Steps of the Light Reactions

by Jason Amores Sumpter
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okay, In this video, we're going to talk about the steps of the light reactions and recall from our previous lesson videos that the light reactions is the first stage of photosynthesis and also recall from our previous lesson videos that the light reactions occur in the file a coid within chloroplasts and recall that the Thilo coins are really just those green pancake looking structures that air within chloroplasts and also recall from our previous lesson videos that if within the Thilo coid membranes, uh, they contain photo systems and recall from our previous lesson videos that photo systems are really just light harvesting complexes that are made up of pigments, proteins and other molecules as well. And so because they are light harvesting complexes, they're gonna be needed for the light reactions. And so if we take a look at our image down below notice that were zoomed into one of the Thilo coins within a chloroplasts because here we're showing you the Thilo coid membrane. So all the way across, this represents the pill, a coid membrane or the membrane of one of those green pancake looking structures within the chloroplast and then down below here. What we have is the Thilo Coid space, which represents the inside of the Thilo Coid or the inside of one of those green pancake looking structures. And then up above. What we have is the Strom A off the chloroplast, which is the space just outside of the fill accords, the fluid filled space that fills up the innermost regions of the chloroplast. And so this is important to note as we make our way through the steps of the light reactions. Now, notice that here in our lesson, we've consolidated the steps of late reaction down to five steps that are labeled a, B, C, D and E, and notice that each of these five steps labeled with these letters corresponds with each of the letters that we have down below for the steps of the light reaction. So that's also important to keep in mind. Now. Also recall from our previous lesson videos that most plants contain two photo systems and those photo systems are conveniently named photo system one and photo system to, And it turns out that these photo systems were named just based on the order of their discovery, and so photo system one was discovered first, so they called it Photo system one and then photo system to was was discovered later on, so they called it photo system to. But it turns out that later they found out that photo system to actually comes before photo system one in the steps of the light reaction. And so it turns out that this entire process starts with photo system to and then photo system One comes later on. And so the very first step of the light reaction step A I's gonna start with photo system to and recall photo systems are light harvesting complexes so they can absorb photons of light. And that's exactly what photo system to does. Photo system to is going to absorb photons of light. And those photons of light contain energy. And so the photo system to its going to absorb the photons of light and transfer that energy to electrons in order to energize those electrons and these electrons that are being energized, they're actually coming from a water molecule. And so these electrons are being donated by a water molecule. And really, this is the entire reason why one of the biggest reasons why water is needed for the process of photosynthesis. And so it turns out that these water molecules are actually going to be split and oxidized in order to provide electrons to this process. And when the water molecules are split and oxidized in order to provide electrons, they're going to react to form oxygen, gas or 02 And really, this is the oxygen gas that's associate ID with photosynthesis producing oxygen. And so let's take a look down below it. Our image and step A to get a better understanding of step A And so once again in the steps of light reaction were zoomed into one of the Thilo coid membrane so noticed that within the Thilo coid membrane, we have these photo systems. We have one photo system here in the second photo system over here. But it turns out that photo system to is actually the photo system that comes first in this process. And so this is gonna be photo system to right here and photo system one. What we'll see is it comes later in the process and step seat when we get there. But taking a look here we can see photo system to is right here and in the first step of light reaction step, a photo system to is going to absorb photons of light. And so you can see that these accessory pigments and the primary pigments are able to absorb photons of light and transfer the energy of those photons of light to electrons, which are being represented here as these circles with these little negative symbols on the inside. And so these electrons are being provided by water molecules, and when water molecules get split in half, they are able to generate oxygen. And this oxygen gas here is really the oxygen that's associated with photosynthesis and photosynthesis producing oxygen. It all comes from the splitting of this water molecule, and also splitting the water molecule also provides the electrons that are going to be used in this process. And so once again, photo system to is going to absorb photons of light and transfer those photons of light, the energy of those photons of light, two electrons and so in step, be the second step of the light reactions. What's going to happen is these energized electrons are going to move from photo system to two photo system one and they're going to do this via an electron transport chain which is really just going to allow for electrons to move from component to component and a Siris of redox reactions. And as these electrons move from photo system to over two photo system one, these electrons are gonna be used to generate a hydrogen ion Grady int or an H plus Grady int or Proton Grady int. And so we'll be able to see that down below in our image of step be And so you can see here in step be these electrons that are here. They're going to make their way over to photos system one over here. And they do that the way that these electrons go from photo system to over the photo system. One, it is via this electron transport chain that we see here. And so in this electron transport chain, the electrons get passed from component to component to component, and those that energy in those energized electrons is gonna be used to pump hydrogen ions into the Thilo Coid space. And so you can see that the Thilo Coid space here is building up. Ah, hydrogen ion concentration Grady int just as we indicated up above. And so ultimately, this hydrogen ion concentration ingredient is gonna be used in a process called Chemie osmosis later on to generate some 80 p. But we're going to continue to follow these electrons here before we jump over to how these hydrogen ions are gonna be used. And so in the next step step See, of course, the electrons have now made their way over to Photo System one and photo system One is also a light harvesting complex, just like photo system to is. And so photo system one is also going to absorb photons of light. And it's going thio take those electrons and energize them even mawr using the photons of light on the energy that it absorbs from those photons of light. And so, photo system one is going to, uh, photo system One electrons are gonna get energized, even mawr, And when they're energized even more, they're going to continue their way through the electron transport chain. And so down below, if we take a look at steps see here, notice again. What we have is photo system one right here, and it is also a light harvesting complex similar to photo system to. And so it's also going to absorb photons of light. And those photons of light are going to contain energy that could be transferred to the electrons reenergizing the electron so that they can make their way through another, uh, continue their way through the electron transport chain. Yeah, and so this leads us into the fourth step of the light reactions here, which is Step D and in this step D. What's going to happen is something called N A. D. P plus is going to serve as the final electron. Except ER and N A. D P plus is really just the oxidized form of N a. D. PH. So it is basically an electron carrier, an electron taxi cab that has to empty seats. And so the N A. D P plus has two empty seats in this electron taxicab, and it's going to service the final electron except er so those electrons, which they started on water. Uh, those water gets split to provide the electrons. The electrons get energized, make their way through an electron transport chain. They get re energized and continued through the electron transport chain and Ultimately, those electrons are going to end up on N a. D P plus and generate an a d. PH. And that's what we're saying here in Step D is that any D P plus serves as the final Elektronik center and is going to be reduced. It is going to be reduced toe form N a. D pH and any DPH is the full electron carrier that's carrying two electrons. And so if we take a look at our step d down below, what we can see is that we're seeing n a d p plus uh, reduction. So N a D P plus is going to be gaining electrons being reduced to n a. D. PH. And that's exactly what we see right here is any DPH is being formed through the reduction of n a. D P plus. And so any D P plus acts as the electron carrier uh, the empty electron taxicab, if you will, and two electrons are going to be transferred to any D P plus to generate any DPH, and ultimately these two electrons are going to originate from the water molecule that was split over here. And so ultimately, the water molecule is providing the electrons that air used in this process and end up on N a. D pH. And so this takes us to the fifth and final step of the light reaction Step E. And in this step, the hydrogen ion, or H plus concentration Grady int that was generated earlier is actually going to be utilized to generate some A T. P. And this 80 p that's gonna be generated is generated via the process of Chemie Oz Moses or the as Moses of ions across a semi permeable membrane. And so here in step e, uh, notice that we have an 80 Pecent, these protein that's embedded right here and this 80 Pecent This is going to allow hydrogen ions to defuse down their concentration. Grady int. And as these hydrogen ions diffuse down their concentration radiant, it's going to energized the fossil relation of a D. P to generate a teepee. And so, ultimately, what we have is some 80 p being generated here in the final step of Kenya's Moses. And so what you can see here is that ultimately, what the light reactions is producing or actually, let's start with the reactions with the with light reaction starts off with the ingredients of the light. Reaction are photons of light and water molecules, and ultimately the water molecules are being split to provide electrons. Uh, the photons of light are going to be utilized to energize those electrons, which allow the electrons to move through the electron transport chain from photo system to to photo System one. And those electrons end up on N a. D. PH. And so, in terms of the products of the light reactions, it ends up forming oxygen, oxygen gas from the splitting of water molecules to provide electrons. It also ends up providing N a. D. PH, and it also ends up providing a teepee and ultimately, the oxygen gas that's produced it can either be utilized by the plant for aerobic cellular respiration, or this oxygen can actually diffuse out of the plant through the stoma of the plant, the openings and leaves and be released into the atmosphere. This oxygen could be released by the plan into the atmosphere, but the n a. D. PH in the A t p that air generated are actually going to be utilized in the second stage of photosynthesis which is the Calvin Cycle. And that's really one of the biggest takeaways here of this lesson is that the light reactions is going to generate N A. D pH and ATP Chemical Energy and the N A. T pH in the 80 p are going to go to the second stage of photosynthesis, which is the Calvin Cycle. And so the Calvin Cycle is going to rely on products produced by the light reactions. And so this year concludes our lesson on the steps of the light reactions of photosynthesis, and we'll be able to get some practice applying the concepts that we've learned here as we move forward in our course, So I'll see you all in our next video.