 Our transition to Photosystem 1 only happens because we need a final electron acceptor for Photosystem 2, for that electron transport chain. So this is Photosystem 1 and interesting, these electrons are going to go, the final electron acceptor is going to be Photosystem 1. That means that as long as we continue to have electrons from Photosystem 2, we're going to continue to run this electron transport chain and continue to produce ATP from the light of the sun. You probably are starting to wonder, wait a minute, how do we replace the electrons in Photosystem 2? Super important question, let's go through this whole process and let's see what happens. Like maybe at the end of Photosystem 1 they go back to Photosystem 2. Let's see what happens. Well, how do you think Photosystem 1 works? I've got great news for you. It's exactly the same. Funnels in light from the sun, exactly the same, same time. La, la, la. Launch, launch, launch. And with the light of the sun, Photosystem 1 creates high energy electrons. La, la, la. Same, same, same. Those high energy electrons are used. I'm going to have to make this electron transport chain a little bit skinnier so I can fit everything in. Same story, the high energy electrons are passed to an electron transport chain. Same story, use the energy to pump, oops, use the energy to pump Hydrogen ions. And look, we're pumping Hydrogen ions into the same Thylakoid lumen. So both Photosystem 2 and Photosystem 1 are creating energy that's pumping, that's forming that Hydrogen ion concentration gradient and which fuels ATP synthase. La, la, la, la, la. Electrons, uh oh. We're going to have a logjam folks. Photosystem 1 needs a final electron acceptor. Who's it going to be? Okay, here are some hints. First of all, the light reactions result in ATP. We've got it. What was the other thing that the light reactions resulted in? I'm gonna give you a hint. Looks like that. Do you know what it is yet? Are you yelling it out? Now do you know what it is? True story, the electrons get dumped into a high energy electron carrier. Those electrons that passed all the way through from the energy of the sun and passed all the way through the electron transport chain, those electrons from Photosystem 1 still have enough energy in them to be high energy electrons carried by a high energy electron carrier. We have our two products, you guys. We've got our high, we've got our ATP and we've got our high energy electron carriers. Those guys are headed to the Calvin cycle. They're gonna provide the energy to fuel glucose production. In cellular respiration, our whole point was producing ATP. So our story ended with this picture. In photosynthesis, we need the ATP. We need the energy in order to do the work of producing glucose. So our story isn't done. We have to send these energy molecules that have now, we've taken the energy from the sun and now we have it well captured in familiar molecules that we know the high energy electron carrier and the ATP. And we're gonna send those guys off to provide the energy. What's the one thing we have not addressed? My picture is very crazy right now. So I feel a little sad that I have to do one more thing. What do we have to do? We've got a very important job. We have to replace, we took the electrons from Photosystem 2 and gave them to Photosystem 1. We then took those electrons and gave them to a high energy electron carrier to go do some work in the Calvin cycle. We're done unless we replace those electrons in Photosystem 2. Where is the energy? Where are those electrons gonna come from? And I guess I will make you a hint. I'm gonna try to make you a hint. Dude forgive me for this because this is a messy picture. That was my hint. What is that? It's water. What do you think might happen with water? It's gonna split. Remember how oxygen captured, like oxygen was our final electron acceptor in cellular respiration and it caught the electrons and then combined with a couple of hydrogens whatever and became water? It's just backwards. This time the water is going to split and become oxygen gas that blows off and electrons that replace the electrons from Photosystem 2 and a couple of hydrogen ions. Yeah, yeah, yeah. Dude aren't you glad you studied cellular respiration first? You tackled all the brain twisting to get cellular respiration to work in your head and because it's all just backwards and but it's the same thing you've seen it before which is so amazing. Oxygen again. Kiss a plant. Let's see what happens in the Calvin cycle when our energy molecules go there.