so at a basic level we all know that plants turn sunlight into food it's the bedrock of our entire ecosystem and food chain in a lot of ways everything we do that requires energy is just solar power with a few extra steps in between but what's amazing to me is that's usually where the science class stops we never make a really important connection and so now let's look back at photosynthesis and answer an important question how do you take sunlight and make it chemically useful in a previous video i took you through how sunlight is initially captured and how plants make oxygen but today let's meet the supporting cast of photosynthesis and see how plants game the system using membranes to start turning that energy into actual food today we'll take a trip down the electron transport chain to photosystem 1 and see all the tiny steps that need to occur to set the stage for the most amazing chemical process on this planet [Music] and real quick this video is part two of a four part series taking a deep dive into every step of photosynthesis if you like going into exhaustive detail learning about the molecular processes that keep us all alive subscribe to this youtube channel so you don't miss parts 3 and 4 of this series in video 1 linked above and in the description we talked about how our first complex confusingly named photosystem 2 captures energy from sunlight by breaking water apart it's cool that we crack open water using sunlight but like how do plants eat that we need a few more steps before this becomes food first up let's remember that our two water molecules have broken apart into one oxygen molecule four electrons and four protons this entire video all at least 12 minutes of it is going to focus on the journey these four electrons go on to start becoming food for our plant now how can four little electrons do all this well it's all in the design of today's system remember this whole thing is happening inside the chloroplast of our plant cell in here we've got a bunch of disc shaped energy converters called thylakoids our electrons are going to turn this thylakoid into a power plant by helping push protons from the outside of the thylakoid called the stroma to the inside called the lumen but like what on earth is that going to do well this video and your understanding of this process hinges on charge our electrons have a negative charge and our protons have a positive charge this works just like a magnet like charges repel each other push each other apart and positive charges attract draw each other together if we shove enough protons into this thylakoid into this lumen space they'll repel each other enough that they'll create a kind of pressure inside our thylakoid the same way water pressure can make electricity in a dam the protons are going to provide the power while our electrons are going to act as bait to attract more protons not only that but our electrons are going to eventually become the ultimate power source to become an actual energy transitioning molecule at the very end of this and so this takes us right back to the end of the last video two water molecules have been completely smashed apart and our electrons have been moved two at a time up to a special molecule called plastoquinone and taking on two electrons plastoquinone has developed a strong negative charge so luckily pq is sticking up into the stroma here because now we can use that negative charge to attract two protons from that stroma once that happens plastic quantum pops off photosystem 2 and our journey begins since this happens twice we've air quotes made four protons in the lumen and removed four from the stroma so now our charge separation is already starting to develop it's plus four in the lumen to minus four in the stroma we're well on our way but our thylakoid doesn't quite have enough power yet it's time for the journey to begin it's time to go down the electron transport chain and once plastic one ohm pops off and gets moving our first stop is your friendly neighbor hood charging complex cytochrome bf i'm going to go ahead and just call this site bf from now on because that's way easier and it almost sounds cute enough to actually stick in your head maybe probably fingers crossed site bf is your classic trans membrane complex you get a bunch of structural protein chains anchoring a total of eight heme groups to the thylakoid membrane like everywhere else you find it these heme groups are the real workhorses of the complex they're arranged in this way because site bf is going to create two different paths for electrons to take our main goal here is to utilize those electrons to charge up the thylakoid lumen even more get as much positive charge out of that stroma and get it into that lumen i'm going to repeat this point so many times it's going to become nauseating and so the main job of cytochrome bf is to accept those electrons from the plastoquinones they're going to hit right here and let's go through that process because this is the most complicated part of the video hang tight we're going to go through some hard chemistry so plastic quinone binds right here and immediately loses both of its protons because it's binding here those protons are going to leave and enter the lumen and so now instead of just removing those protons from the stroma we've added them to the lumen but from this point onward our electrons are going to bifurcate they're going to take two separate paths half of them are going to move down and attach to a new karyo plastocyanin one at a time the other half are going to move up and become bait to attract more protons but again one at a time so let's look up first our one electron travels from the initial binding point up a bunch of heme groups to this topmost one and here it's going to make this heme group just negative enough to attract another free plastoquinone from the stroma that pq takes on that electron and has half a negative charge that's nice but not quite enough luckily remember ps2 made two of these plastic quinone bundles another pq is going to hit site bf in the exact same spot and do the exact same thing its electrons will bifurcate its protons will enter the lumen but this second electron is going to have a special job because it's going to complete the charge on this stromal plastoquinone and so the exact same thing that happened in photosystem 2 was going to happen now this plastic quantum has a negative 2 charge so it's going to take up two protons from the stroma and so in just transferring our electrons over we've actually managed to remove two more protons from that strum up making its minus charge even bigger while at the same time our plus charge on the lumen also got much larger now this plastic quinoa i'm just going to dissolve into the thylakoid membrane and go off to hit another cytochrome bf somewhere else and have its electrons bifurcate so it's just helping game the system a little bit we air quotes sacrificed two of our electrons to do this but now we have a charge separation great enough to power the engine at the end of photosynthesis more on that in video three if that's out by now it's popping up but to get back to the actual journey while half of our electrons go through this weird little cycle where they go up become plastic one and again and then re-enter the system half of them are going to continue the journey and so if they move down they have bound to this protein plastocyanin and so we move on to plastocyanin now we're going to use these last two electrons to actually store a little bit of energy in the stroma and power part of the dark reactions of photosynthesis i.e video four but here's the thing all the jumping that these electrons have done they've lost a little energy throughout that process that energy went toward a good cause though it went toward cramming as many protons as possible into the lumen but in order to complete this journey we're going to need a little bit more energy and we've got just the tool we need to get it let's light these electrons up with sunlight one more time with our last complex photo system one again it's a little goofy that this journey starts at ps2 and ends at ps1 but we work with the labels we have just remember that ps1 only has that title because it was the first of the two to be discovered and ps1 is remarkably similar to ps2 it's got that same general idea ps1 is a little bit larger than ps2 it's a trimer not a dimer i.e it has three subunits instead of just two and it doesn't have an oxygen evolving center it's still got the light harvesting pigments that are held together by these little protein chains and a special pair of chlorophyll molecules at the bottom that turn all this energy into something useful but these two chlorophylls are arranged a little bit differently they absorb light at a slightly redder light than ps2 this pair is called p700 because it absorbs light best at a wavelength of 700 nanometers that light either gets directly absorbed the energy will cascade down from one of ps1's antenna complexes now photosystem one works in a remarkably similar way to photosystem 2. ps1's reaction center gets excited by absorbing solar energy and throws an energized electron up a series of acceptors until it hits our final ferry up here ferrodoxin we'll come back to that in a second meanwhile the p700 reaction center is now violently oxidized and is ready to rip electrons off of anything nearby which is convenient because our plastocyanin is delivering fresh electrons from site bf right as this happens our charges now rebounds until the next photon hits and our electrons are now excited enough to do one last little bit of cellular energy work and the cycle goes on and on but back up to the final steps where does ferrodoxin go and what is the ultimate fate of our electron travelers and finally we're getting to the end of this journey here with our last carrier ferrodoxin varidoxin carries this electron to its final destination in the stroma all fd is going to bind to this enzyme where the electrons can get bound to a familiar energy molecule nadph nadph is one of those really important molecules that allows living things to move energy around their cells you might be familiar with nadph's more popular cousin atp we're gonna finally be talking a lot about atp in our next photosynthesis video but for now we're turning nadp plus into nadph please note that nadp plus needs a hydrogen atom to turn into nadph you know hence the h but that hydrogen atom has a positive charge so we actually need two electrons to pull this off so we go through two plastic quinones at site bf to get plastocyanin to give two electrons to ps1 which will get bound up into one of these nadph molecules and so that's it sunlight has done its job for now and the stage is set for all of this potential energy to become pure chemical energy in video three we're going to talk about what these protons are doing and what kind of energy they're going to actually turn into and then we're going to talk about exactly what nadph is doing in that fourth video once all that energy comes together and crystallizes air into food itself this is the pure joy of studying biology for me you go through this video a bunch of times and understand the mechanics of how sunlight is genuinely turned into chemical energy you get an astonishing sense of wonder for how well coordinated this dance is this only works because of how well designed these transmembrane proteins are and how well choreographed our electron carriers are what i want you to take away from this is just how important the structure here is this is why you're forced to memorize biology because if you memorize the structure you begin to understand the function either way i'm really excited for the stage that's set now next video we're going to be talking about the 7 800 rpm engine that uses all of this energy to make the most important cut molecule on earth adenosine triphosphate atp i can't wait to share it with you but for now thank you so much for your time and as always thank you so much for watching to the very end of this video if you like this at all please a go check out our first video on photosynthesis about photosystem 2 and also please like comment and subscribe we're doing a really great job of developing momentum on this channel i want to make sure it has the best chance to grow as possible if you liked this and want to see more of it i have a huge editorial plan that's going to get me across the next couple of years honestly but the only way that's going to actually happen is if we get support directly on patreon so i really encourage you to check that out link in the description at the same time i've cited all of my sources and am taking discussion over at my twitter at this underscore clockwork feel free to check that out there's a link to the actual thread in the description as well either way i really appreciate your time but as always i'd like to leave you with peace love and plastician and everyone be well thank you so much you