Okay, let's move on. Light reactions. So before we move on to the light reactions, I just want to put a little warning out there. This is where we're going to get into the more difficult topics. Most of my students tell me that units three and six are the hardest, and we're on unit three right now.
I'm going to try and make this hopefully a little easier for you when we go through this, but just know it's going to take... several times of reviewing and listening and watching videos and doing our experiments and doing practice problems to really grasp what's going on. I would never expect one of my students to be an expert at this right when you finish this video, let's say. You're probably going to have questions. You're definitely going to need to go back to your text to review, and I'll give you some practice videos that you can watch to help you as you go through too.
But make sure that you're asking questions. in class so that we can address everything that comes up. So let's go ahead and start the light reactions.
Here's an overview. So again, we're going to be in the thylakoids because the light reactions happen in the thylakoids. So for the overview, again, it's going to occur in the thylakoid membrane in the photosystems, and we'll talk about what the photosystems are.
But over here, we can see our little overview. So we have light and we have water. It's entering into the thylakoids.
And then we're going to release ATP, NADPH, and oxygen. Now this one down here is sort of the one that students see and you're frightened a little bit, right, because it looks really complex. And just so you know, you don't need to memorize any of these names that you see down here besides photosystems and ATP synthase, which is down here. And again, I'll go through all of this as we go through it. So hopefully that helps relieve some of your stress.
You don't have to memorize this step by step. However, you do need to understand the steps in order to understand why this happens. So while you don't need to memorize lots of names, you do need to understand what's happening in each step. So let's go ahead and go on to the next point here, which is the conversion of energy. So we have this huge conversion, right?
We take in All of this solar energy, or why am I saying we? Humans don't do it. I wish we could.
Can you imagine how cool it'd be if humans could go through photosynthesis? Just think about that. But no, we don't.
Plants do. So plants take in all of this solar energy and they're going to convert it to chemical energy. So it's going to occur in the thylakoids and we're converting energy, a major energy conversion here in the light dependent reactions. All right, so light reactions. Here we go.
Chemical energy is going to be in two forms. We're going to have NADPH and we're going to have ATP. We know what ATP is.
Remember, we talked about that before. That's adenosine triphosphate, and we know it's an energy form for the cells. So how does the cell accomplish this? Well, the cell accomplishes this conversion by using light energy to excite electrons. So light energy, we can see two areas of light energy right here.
So here's light energy and here's light energy. And for both, it's going to be exciting electrons. So why is light actually important to chlorophyll then? What's it doing?
Well, remember we said pigments absorb light energy. So chlorophyll itself is going to absorb a photon of light. So down here, I have this little image to represent our chlorophyll molecule.
We have a photon of light coming in. And what's going to happen then is it's going to Boost electrons, and we'll talk about where these electrons come from in a minute, that's going to boost electrons from a ground state to an excited state. When this happens, the electron is unstable, and it's going to fall back to the ground state. And when it does, it's going to release energy as heat, and it's going to emit photons as fluorescence.
So plants actually do go through a little bit of fluorescence because of this. So we need light. and we need chlorophyll in order to excite these electrons.
And we're going to tie it into redox reactions in a minute. So we have to have light in order for these electrons to go from ground state to an excited state. Now what are photosystems?
Because on this slide here I mentioned Where was it? The previous slide, I think. The one before that.
I mentioned photosystems. So what are these? Well, photosystems are embedded in our membrane.
And again, I keep saying our. I need to stop doing that. They're embedded in a plant's membrane, okay?
Plant's membranes. And they're reaction centers, and they're light-capturing complexes. So what's a reaction center?
Well, a reaction center is a complex of proteins associated with chlorophyll, and something that can accept an electron known as an electron acceptor. Within photo systems, we also have light capturing complexes. And these are pigments or mirror pigments can absorb light energy. So they're in there and they're they're capturing all of the light that's associated or that I should say is in our photo system. So with the reaction center, they're working together and order to capture light.
One way to think of this is like an antenna for the reaction center. So the pigments are going to be capturing a lot of light and sort of funneling all of that into the reaction center. In our thylakoid membrane, there are two photosystems.
We have photosystem 2 and we have photosystem 1. Why am I going out of order? Well, it's because photosystem 1 was discovered first, so it was named first. But notice in our thylakoid membrane, photosystem 2 actually comes first. So it's going to be preceding photosystem 1. So photosystem 2 is known as the reaction center P680. And that's because it absorbs light at 680. nanometers.
Photosystem 1 is known as reaction center P700 and that's because it absorbs light at 700 nanometers. All right like I mentioned these are named in their order of discovery. So photosystem 1 was discovered first and that's why it's named first. Okay, now what we're going to do is delve into these photosystems. So you don't need to write anything down here, but over the next few slides, we're going to look into this thylakoid membrane, and we're going to delve much deeper into the photosystems, as well as some of the other proteins here, so that we can see what's actually happening.
So this is the big picture. Now let's start going step by step with what's happening. All right, so we're going to start with photosystem two, because it's first in our thylakoid membrane.
So we know that light is going to sort of drive this. So we have light coming in and when light comes in, it's going to cause an electron to go from an excited state back to a ground state. And this is going to repeat until it reaches the P680 pair of chlorophyll molecules. So let's watch what happens. So here comes light.
So go ahead, look at the screen. Here comes light. And we're causing all of these pigment molecules to pick up on this light energy. And we're having electrons going from a ground state to an excited state, ground state to excited state, until it reaches P680. All right, at this point in time, our electron is going to be transferred to a primary electron acceptor, which means that we're going to go from P680 to P680.
P680 plus because now it lost an electron. And inside of this photo system, we have our first primary electron acceptor. At the same time, we're having water. Water's coming in and it is splitting.
So water is split to two H plus and then half O. two, but you always write it O2. So we have two hydrogens and one oxygen, but you write it like this.
So at the same time, water is splitting. So water is split into two electrons, reduces 680 plus, and then two hydrogen ions. These hydrogen ions are going to be released into the thylakoid space.
Okay. So we have two electrons. It's reducing P680 plus and we have two H plus that are released into the thylakoid space. Now, the one oxygen atom will immediately bind to another oxygen atom forming O2, forming O2. Now, the flow of electrons through this electron transfer system is leading to this H plus being released into the thylakoid space.
So this H plus that's going into the thylakoid space It's actually creating a gradient across our inner membrane. This is known, like I said, as a linear electron flow or linear flow of electrons. So each excited electron will pass from photosystem two to photosystem one through an electron transport chain or ETC, electron transport chain or ETC. All right, so there is our electron, and now it's going to go through a series of redox reactions. So first, the electron is going to be transferred to plastoquinone, then it's going to go to cytochrome, and then it's going to go to plastocyanin.
Now, this is where I said you don't need to know some of the names. It's not required for the AP exam or for this course, and this is what I was talking about. So you don't need to know the name of plastoquinone or cytochrome or plastocyanin. But what you do need to know is that they are participating in redox reactions.
So for example, when plasoquinone gains the electron, it's been reduced. But then plasoquinone is going to give that electron to cytochrome. Therefore, it becomes oxidized.
And then cytochrome becomes reduced. And then cytochrome passes the electron to plasocyanin. So cytochrome becomes oxidized.
And then plasocyanin becomes reduced. Right. So we have this series of... oxidation reduction reactions or the redox reactions happening in this linear electron flow.
Now this linear electron flow is also going to contribute to the hydrogen ion gradient. So as this electron passes from electron acceptor to electron acceptor, we're actually pumping some more. It's powering the pumping of more hydrogen ions into the thylakoid space.
And that's going to be important in a minute. All right, so now let's talk about the generation of ATP. So like I mentioned, this fall of electrons going from photosystem 1 to photo, excuse me, photosystem 2 to photosystem 1 provides energy to form ATP.
Because anytime we have a gradient, we talked about this a little bit in unit 2, remember, this hydrogen ion gradient is a form of potential energy, which means we can transfer it or make it go into kinetic energy. And we'll use that in the ATP synthase. So ATP synthase is going to couple the diffusion of hydrogen ions to the formation of ATP.
So let's just take a second and review where we're at. So light, so I'm down here. So light has gone into photosystem two.
At the same time, water is being split. When water splits, we have oxygen and we have hydrogen ions. Electrons are being excited up in PS2. And then look, we're going through a series of redox reactions.
It goes to plastoquinone, to cytochrome, to plastocyanin. And look at this hydrogen ion. What's happening?
Notice it's getting pumped through. So inside of our thylakoid lumen, we have a really high concentration of hydrogen ions. And that gradient is going to allow for the formation of ATP, which is what we're going to get into next. So we left off here at plasocyanin. Now we're going to pick up what happens after that.
So here we are, right? Plasocyanin. So light energy, again, every photosystem takes in, or each, I should say, photosystem takes in light energy.
So now we're at photosystem one. So photosystem one is capturing light energy, and it's going to excite the electrons in the P700 chlorophyll molecules. They'll become P700 plus, right?
Because they're losing it. There we go, and it's going to go to a primary electron acceptor. So that light energy will excite the electrons in the P700 chlorophyll molecules. They'll become P700+.
They're taken to a primary electron acceptor. At this point in time, the electrons go down a second transport chain, so we have some more redox reactions. And here is where we're going to get into the next really important part. So the first really important part is that water is split. And when water is split, it's providing electrons that are flowing, right?
The electrons that we see flowing here through our thylakoid membrane. It's providing that the hydrogen in the thylakoid lumen, that's going to power ATP synthesis. And here's the next part. NADP plus reductase, an enzyme, it's going to catalyze a transfer of electrons from ferredoxin to NADP+.
So notice we have two electrons total right here. These two electrons are going to be going through a reaction with NADP plus reductase. So what will happen here is we have NADP plus, plus our electrons and hydrogen ion, and we're going to get NADPH.
All right, so we said at the very beginning with the overview of the light-dependent reactions that we produce two things. We produce ATP and we produce NADPH. Here is the first thing we produce, NADPH. So this is the first really important thing that's being produced.
Okay, so what happens from here? So now we've made it to our ferredoxin. We have NADP plus reductase.
So it takes NADP plus, plus our electrons, plus our hydrogen ions, and we get NADPH. All right, so now let's go ahead and put this all together. And we're going to talk about this ATP synthase a little bit more.
So let's review. Light entered, photosystem 2. Water split. When water split, we got oxygen and we got... our hydrogen ions forming a gradient.
Our electrons were excited because we have pigments in here that are picking up on that light energy. And we see redox reactions that are going to take our electron all the way over to NADP plus. And NADP plus, plus our electrons, plus hydrogen ions are going to give us NADPH. All right, so that's the first thing that we produce. First important thing.
Second thing. this hydrogen ion gradient. So we have lots of hydrogen ions, remember, in the thylakoid membrane. They're going to be taken over here to ATP synthase, and they're going to pass through. When it passes through, we go from ADP plus our inorganic phosphate to ATP.
And that is how the hydrogen ion gradient powers ATP synthase. And here is the second major thing that we produce here in the light reaction. So what happened so far?
We got light energy, we split water, we got NADPH, and we got ATP. Now remember, the hydrogen ion gradient we said was a form of potential energy. Potential energy here was then transferred to kinetic energy because it's almost like a rotor-like structure.
This rotor-like structure here on ATP synthase actually spins, it moves. And when it moves, that's how we get ADP to ATP. P, adenosine triphosphate. Okay, let's put that into a list of inputs and outputs.
So what did we need for the light reaction to happen? Well, we needed water, we needed ADP, and we needed NADP+. What were the outputs?
Well, we got oxygen. Oxygen is just a byproduct. Remember, it's not actually used anywhere. If you notice, it wasn't used anywhere.
It's just a byproduct. The cell actually does not need it. So it gets rid of oxygen.
We got ATP and then we got NADPH. So again, oxygen is just a byproduct. And it came specifically from the splitting of water.
So it's interesting, right, that it's so important for us because we breathe oxygen. But for the plant, it's just junk, right? It needs electrons and hydrogen from water. And then it just gets rid of oxygen as a byproduct. Act.
Okay, now the second main step of photosynthesis is the Calvin cycle. And we're going to notice that these two right here, ATP and NADPH, go straight into the Calvin cycle. So the Calvin cycle actually needs these to function.
And then we'll find out once we go through the Calvin cycle, if you were wondering where does ADP and NADP plus come from, guess what? It comes from the Calvin cycle. So the light.
dependent reactions and the Calvin cycle work together in order to function properly. So the Calvin cycle provides ADP and NADP plus and right water comes from soil. Okay.
And then the Calvin cycle needs ATP and NADPH. So they work together cooperatively. All right. So let's put our light reaction summary into words.
So here we're doing a major conversion, right? We're going from solar energy. to chemical energy. And the chemical energy was in two forms, NADPH and ATP.
And again, we know now that these are going to be used in the Kelvin cycle. So the Kelvin cycle is going to use these. Okay, other major thing, water was split. This provided a source of electrons and protons and oxygen was released as a byproduct.
When we had light coming in, Our pigment chlorophyll absorbed that light, and that's what actually drove the transfer of electrons and hydrogen ions from water to the electron acceptor NADP+. So we went, well, we went through lots of electron acceptors, but the main one was we went from H2O, we got our electrons, and we got our hydrogen ions, and they went to NADP+. Why? Because then we got NADPH.
So that's how we got it. So It was this transfer that way. All right, like I said, NADP plus is reduced to NADPH, and then the hydrogen ion gradient allowed for the generation of ATP by phosphorylating ADP.
And again, we did this through ATP synthase. So the hydrogen ions flowed through ATP synthase. And then that's how we generated ATP.
Now the process of using this hydrogen ion gradient to power ATP synthesis is also known as chemiosmosis. So I'm going to write that here. I suggest you add it to your notes just so you know. It's also known as chemiosmosis. Sometimes you'll hear that.
Okay, so again, this process of creating ATP. is also known as chemiosmosis. Okay, so the overall yield of this linear electron flow pathway is one NADPH and one ATP for each pair of electrons produced from splitting water.
So for each pair of electrons from splitting water, we got one NADPH and one ATP.