Hi in this video, I'm gonna be talking about the light dependent reactions of photosynthesis. So if we're going to talk about the light dependent reactions, we need to really understand what the driving proteins are behind these reactions. So the protein that we need to know most about is called a photo system. And so what is the photo system? That's gonna just be protein complexes? And there are two photos two photo systems. But each one is a protein complex where light dependent reactions take place. So where is it? And what is it? What does it consist of? Well, the first thing is photo systems are found in the silo coid membrane and they contain a bunch of different regions that we're just going to have to learn the vocabulary for. So the first is going to be the light harvesting center. You may also see this as the antenna complex. And this part portion of the photo system is responsible for taking in that light energy and turning it into electrical energy. So this process is called photo excitation. Which is when light energy excites an electron and then that electron can be used to turn into something else. But the electronic self is electrical energy, The excited electron itself. Then you have the second region of the photo system and that's called a reaction center. And so this takes in that electron that electrical energy and transfers it to chemical energy. So now we're doing light to electrical energy. We're doing electrical energy to chemical energy. So those are the two different energy transfers in the two different regions in the photo system. So the important thing you need to know is that in order to take in light, the photo system needs some type of pigment that can absorb it. And so this pigment we're all familiar with, this is chlorophyll pigment um and that is going to accept the light. Um and it sound in the chloroplasts. And so the reason that chlorophyll can accept light, you don't really think about, you know why chlorophyll can take in light, but it can take in light because it has this unique chemical structure. Um It has this like light absorbing ring um if you're just interested in what that ring is called, it's called por fin ring, but you don't necessarily need to know about. Um and essentially this ring has easily excitable excitable electrons. These electrons when they're um you know, I can just be very easily excited by light. And when they are excited, they want to release that energy, like very quickly they have all this energy and they want to get rid of it. And so the photo system during photosynthesis uses that excited electron the energy from that excited electron to do a lot of things by converting that energy into different energy forms. And so electrons are really the driving force um in the photo systems and through photosynthesis um and they travel between the two photo systems and other protein complexes um mainly through what we've talked about before are electron carriers. So things that can carry electrons, high energy electrons to other things. So let's look at what a photo system looks like. So we have our two regions here. We have our light harvesting complex and we have our reaction center. And you can see that there's a lot of core field molecules here. And what happens is that the light comes in hits a chlorophyll molecule that excites an electron. That energy is so so so so happy. It jumps around, moves to the reaction center and that. So this is the light energy transferring to electrical energy. And then in the reaction center that electrical energy gets eventually transferred into chemical energy. We're going to go over every single step throughout this entire process of how each of these energy transfers happens. But this is what a photo system looks like. And this is why it's driving photosynthesis. So now let's turn the page.

Okay, so in this video, we're gonna be going through every single step that happens in the light dependent reactions. Um like in previous video, cell biology is a lot of just sort of memorizing steps of how things happen. So hopefully I've displayed these steps very clearly. And so let's now go over them. So the first thing that happens in photosynthesis and the light dependent reaction steps of photosynthesis is that a light photon comes in and it enters into the light harvesting region of photo system too. So we may think that it's weird that photo System Two is actually found in the first step, but that's because photo System Two was actually discovered second. Um So it's kind of this like weird miss naming situation, but because this one was found second, it's called Photo System Two, but it actually happens in Step one. So the light photon comes in into Photo system two and it hits a chlorophyll molecule. Now, once it hits the chlorophyll molecule that is going to excite an electron. And so once that electron is excited, it wants to do something with that energy, it wants to kind of get rid of that energy. And so how it does this is it actually transfers that energy through all the different adjacent chlorophyll molecules. Now this is important because this is called resonant energy transfer because it's transferring the energy, it's not transferring the electronic self. So that's a big important, not the electron is jumping through chlorophyll molecules instead, it's just the energy. And so um in this process it's just bound that energy is bouncing around between chlorophyll molecules. Now eventually just really through chance, eventually this energy is going to reach this special pair of electrons. And so a photo system to these electrons are called the P680 and the repair. So there's two of them. So that energy is bouncing around all these chloroform molecules. And it finally gets to the special pair. And so what happens is once that energy is inside that special pair, that pair will actually transfer an electron. So one electron is actually going to be transferred to an electron carrier called plastic quit Queenan. And that the name changes when it has an electron because it's technically different because it has an extra electron. So it becomes plastic one all. So um so now what we have is this energy has been jumped around through all these core form molecules reached a special pair. And then that special pair has donated an electron very excited electron to these electron carriers. Now the special pair has to be a pair of electrons but it just gave one away. So now it's just one electron. But it has to be a special pair. So it needs to have another electron come in. So where does that electron come from? That replacement electron is going to come from splitting water. And that process is called uh analysis. And in this process, oxygen is released. And so um the we talked about this electron jumping. Um and that electron is actually donated. Well the thing that is going to accept that electron, it's called the primary electron except er which makes complete sense. So let's go through the steps of this one by one. I've labeled them appropriately here. So in step one you have your light photon coming in and hitting a chlorophyll molecule which is here laura. Now this process is gonna be when the energy is jumping through all these different chlorophyll molecules but eventually it reaches this special pair back out so I can write. And this is different because in this process which is step to the electron itself, not just the energy is going to be transferred to the primary except er um primary electronic sector. Then we're missing now an electron so one electron is gone. So it needs to be replaced. So in step three water comes in and gives some electrons and oxygen is going to be released. And so those are the first three steps of the light dependent reactions. Now, what we have is we have this really excited electron being carried in this electron carrier plastic one. All and so that is going to want to transfer it to something. So the next thing that it transfers to is called an electron transport system. Now this is really similar to the electron transport chain that we talked about in um oxidative possible relation in the mitochondria. But this is happening photosynthesis chloroplast. So it's gonna have a different name. So this is called the electron transport system. And in this system. So uh so let's look at the image while we talk about it. So remember this is photo System two and electron came in. It's going into placed O'Quinn in then that is going to be transferred to what's known as the cytochrome B six F complex. And so um now the site of con V six F complex has this excited, really excited electron. And so it begins to use that energy and it uses that energy to pump hydrogen protons into the silo coid space. So again like oxidative false relation. This energy is being used to pump hydrogen protons create this electrochemical gradient. So after that we have a now non excited. So here we have this really excited electron and here we have this non excited electron because this complex here use that energy to pump hydrogen protons. Now, after this cytochrome complex is done with that electron, it's going to transfer it again to what's known as plaszow sign, in which have abbreviated pc here. So now what happens? Well now we move to photo system one. So no photo system one is gonna be step five because it actually occurs second in this thing in these steps. And so what happens is photo system one pretty much the exact same as photo system too. So, step five, a light photon comes into photo system one and hits a chlorophyll molecule. Now the energy from that excited electron is going to bounce between many chlorophylls. Not the electronic self, just the energy Once that energy transfers to a special pair which is called something different. This is called p. because its photo system one. Um Then that special pair transfers one electron um to the primary electronic sector which in this case is going to be fair toxin. And so um which will eventually transfer to paradox in then that donate then that electron that's been lost and the special pair needs to be replaced. So what is it replaced by is replaced from the electron that travel through photo System two through the cytochrome complex to place a sign in and now comes into the special pair. So if we're looking at this um labeled it here. So Step five, what we see His light comes in, bounces through different core film molecules. Then in step six this energy travels to the special pair. This is gonna be P- 700 back out. Now once it's in the special pair, the actual electron is going to be transferred to the primary except er and eventually transferred to fair docks in the electron carrier. And so in um now the donated electron is replaced from here from class to sign in that traveled through the cytochrome complex from photo system two. And this is photo system one. So it's it's a lot of steps but kind of the first part that happens in photo system two is replace happens again in photo System one. The only difference between, you know what happens in Photo System Two and Photo System One is the name of the special pair. Just P 6 80 or P 700. And where the electron comes from, that's going to replace the special pair. So in photo System two, that's going to come from splitting water. And in photo System one that's going to come from the photo system two electron. So that's those are the two main differences between these first groupings. So then let me come back then we have seven H. So now we have paired oxen which is carrying this electron from photo System one and it carries the electron to what's known as N. A. D. Plus reductase. And what this does is it forms in a D. P. H. In this troma. And we need this because it's going to be used in the next steps which are the light independent reactions. So what we get is this electron comes in, travels through the N. A. D. P. Um plus reductase and that ends up producing N. A. D. P. H. Plus hydrogen protons. And then finally in step nine. What we see is through this entire process we've been pumping hydrogen protons. And so that creates a hydrogen proton gradient or electrochemical gradient and that is used in a very similar way by a T. B. Sent face and forms a T. P. So if we're just gonna do an overview of photosynthesis, what we see is that this is the silo coid lumen. So this is the silo coid lock oid membrane. So what happens is we first the light comes into photo system to that energy is going to jump jump jump around into different things but eventually it's going to reach a special pair called P 6 80. That's gonna donate an electron and it's going to travel to plast oh qanon. That electron is replaced by water. Now, plastic winnin transfers this to the cytochrome B six F complex which uses that energy to pump hydrogen. Then that unexcited electron now goes to plaza signing from here, the light comes in to photo system one, it jumps around, ups around the energy jumps around until eventually it excites an electron in the P 700. Now the P 700 is then going to donate an electron which eventually goes to fair dioxin and that's going to be replaced by the electron carried by plasticine in Now that paradox in has this electron is going to go through the N A D. P A N A. D. P. Plus reductase which is going to form N A. D. P. H. And this whole time we've been making hydrogen gradients and this hydrogen, it creates this electrochemical gradient here which in step nine is going to flow through a T. P synthesis to create a teepee. So it is a lot, but it does kind of make sense. It flows through this step where light comes in, electrons jump around, electron donates, travels down, travels through pumps, more hydrogen keeps traveling through. So each one of these steps is just, you know, just this sort of very linear process of electron movement through various protein complexes, mainly through the photo systems that lead to the production of n A. D. P. H and a T. P. So that's the overview of those are the nitty gritty of the light dependent reaction. So with that, let's now move on.

Okay, so in this video we're gonna talk about the cyclic um light dependent reactions. So previously, when we're talking about all those different steps, it was a very linear process. It just sort of traveled between protein complexes all the way down the chain. But in a cyclic light dependent reactions, there is this cycle where it circles back and it does something different. So the cyclic light dependent reactions uh that process creates A T. P. But not in A. D. P. H. And so the reason it does this is because Photo System Two is going to work the exact same in the linear pathway. So the first few steps of that pathway that we painstakingly went over works exactly the same. The difference in cyclic light dependent reactions occurs in photo System One which instead of transferring um its electron to what's known as the N. A. D. P. Plus reductase, it instead goes back to the side of your own complex. So let me show you this image while I talk about it. So this is Photo System One. So this is gonna be the second half of the light dependent reactions. So this light comes in, it jumps around, it eventually ends up in the special pair P. 700. And that donates an electron to the primary electronics sector. And we get to fair docks in. Now, normally in the linear pathway that's going to go through N. A. D. P plus reductase in the linear pathway. But instead we want to learn what's going on in the cyclic pathway. So what happens in the cyclic pathway is instead of doing this, it doesn't do this instead cycles back through to before and goes through the cytochrome complex and this ends up in the production of a teepee. And then that electron is replaced pretty much by itself. It just cycles back through that same exact electron. It gets excited. That energy is used to create A T. P. Instead of N. A. D. P. H. And then it cycles back through um and replaces itself. And so yeah, so that is the cyclic reactions which are different and use much more rarely. Like this is if the sale needs more 80 P. But not in A. D. P. H. But the more typical pathway is the one that we talked about, which is going to be the linear pathway. But this pathway can happen. Um So I need to tell you about it. So that's the cyclic pathway. Let's now move on.