Light Dependent Reactions: Study with Video Lessons, Practice Problems & Examples

Photosynthesis involves light-dependent reactions occurring in the thylakoid membrane, primarily through two photosystems: Photosystem II (PSII) and Photosystem I (PSI). Light energy excites electrons in chlorophyll, initiating a series of energy transfers. PSII donates an electron to plastoquinone, which then moves through the electron transport system, creating a proton gradient used by ATP synthase to produce ATP. PSI, upon absorbing light, transfers electrons to ferredoxin, leading to NADPH formation. Cyclic reactions can occur, generating ATP without NADPH when needed. Key processes include photoexcitation, resonance energy transfer, and chemiosmosis.

Photosystem Structure

Hi. In this video, I'm going to 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 photosystem. And so what is a photosystem? That is going to just be protein complexes, and there are 2 photosynthesis 2 photosystems, but each one is a protein complex, where light-dependent reactions take place. So where is it and what does it consist of? Well, the first thing is photosystems are found in the thylakoid 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 portion of the photosystem is responsible for taking in that light energy and turning it into electrical energy. So, this process is called photoexcitation, which is when light energy excites an electron and then that electron can be used to turn into something else, but the electron itself is electrical energy, the excited electron itself. Then you have the second region of the photosystem 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 kind of the 2 different energy transfers. The important thing you need to know is that in order to take in light, the photosystem needs some type of pigment that can absorb it. And, so, this pigment we're all familiar with, this is chlorophyll pigment, and that is going to accept the light, and is found in the chloroplast. 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. It has this light-absorbing ring. If you're just interested in what that ring is called, it's called a porphyrin ring, but you don't necessarily need to know that. And, essentially, this ring has, easily excitable electrons, so these electrons, when they're, you know, 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 photosystem 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 in the photosystems and through photosynthesis, and they travel between the 2 photosystems and other protein complexes, mainly through what we've talked about before, our electron carriers. So things that can carry electrons, high-energy electrons to other things. So, let's look at what a photosystem looks like. So, we have our 2 regions here. We have our light-harvesting complex, and we have our reaction center. And you can see that there's a lot of chlorophyll 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 happy. It jumps around, moves to the reaction center, and that, so this is the, light energy transferring to an 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 photosystem looks like, and this is what is driving photosynthesis. So now let's turn the page.

Steps

Okay. So in this video, we're going to be going through every single step that happens in the light dependent reactions. Like in previous videos, 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.

The first thing that happens in photosynthesis, in the light dependent reaction steps of photosynthesis, is that a light photon comes in and it enters into the light harvesting region. Photosystem 2 is actually found in the first step, but that's because Photosystem 2 was actually discovered second. So it's kind of this, like, weird misnaming situation, but because this one was found second, it's called Photosystem 2, but it actually happens in step 1. So the light photon comes into Photosystem 2, and it hits a chlorophyll molecule. 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 resonance energy transfer, because it's transferring the energy; it's not transferring the electron itself. So that's a big point; not the electron is jumping through chlorophyll molecules. Instead, it's just the energy. And, so, in this process, this 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 in Photosystem 2, these electrons are called the P680, and they're a pair, so there are 2 of them. So that energy is bouncing around all these chlorophyll 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 plastoquinone, and that the name changes when it has an electron because it's technically different because it has an extra electron, so it comes becomes plastoquinol.

So, now what we have is this energy has been jumped around through all these chlorophyll 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 photolysis. And, in this process, oxygen is released. And so, we so we talked about this electron jumping, and that electron is actually donated. Well, the thing that is going to accept that electron is called the primary electron acceptor, which makes complete sense.

So let's go through the steps of this, 1 by 1, I've labeled them appropriately here. So in step 1, you have your light photon coming in and hitting a chlorophyll molecule, which is here. Chlorophyll. Now this process is going to be when the energy is jumping through all these different chlorophyll molecules. But eventually, it reaches this special pair. Let me back out so I can write. And this is different because in this process, which is step 2, the electron itself, not just the energy, is going to be transferred to the primary acceptor, primary electron acceptor. Then, we're missing now an electron, so one electron is gone, so it needs to be replaced. So, in step 3, 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, plastoquinol. 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 oxidative phosphorylation in the mitochondria, but this is happening in photosynthesis, in chloroplasts, so it's going to have a different name. So this is called the electron transport system. And in this system, so, so let's look at the image while we talk about it.

So remember, this is Photosystem 2, and electron came in, it is going into plastoquinone, then that is going to be transferred to what is known as the cytochrome B 6F complex. And so, now the cytochrome b 6f 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 thylakoid space. So, again, like oxidative phosphorylation, this energy is being used to pump hydrogen protons, create this electrochemical gradient.

So, after that, we have a now non-excited electron. So, here we have this really excited electron, and here we have this non excited electron, because this complex here used that energy to pump hydrogen protons. Now, after this, the cytochrome complex is done with that electron, it's going to transfer it again to, what's known as plastocyanin, which I have abbreviated PC here.

So now, what happens? Well, now we move to Photosystem 1. So, no, Photosystem 1 is going to be step 5, because it actually occurs second in this process. And, so, what happens is Photosystem 1 acts pretty much the exact same as Photosystem 2. So, step 5, a light photon comes into Photosystem 1 and hits a chlorophyll molecule. Now, the energy from that excited electron is going to bounce between many chlorophylls, not the electron itself, just the energy. Once that energy transfers to a special pair, which is called something different, this is called P700 because it's Photosystem 1, then that special pair transfers one electron, to the primary electron acceptor, which in this case is going to be ferredoxin. And so, which will eventually transfer to ferredoxin. Then, that electron that's been lost in the special pair needs to be replaced. So what is it replaced by? It's replaced from the electron that traveled through Photosystem 2, through the cytochrome complex, to plastocyanin, and now comes into the special pair.

So, if we're looking at this, I've labeled it here, so step 5, what we see is light comes in, bounces through different chlorophyll molecules, Then, in step 6, this energy travels to this special pair, this is going to be P700. Now once it's in the special pair, the actual electron is going to be transferred to the primary acceptor and eventually transfer to ferredoxin, which is the electron carrier. And so, now the donated electron is replaced from here, from plastocyanin that traveled through the cytochrome complex from Photosystem 2, and this is Photosystem 1.

So it's a lot of steps, but kind of the first part that happens in Photosystem 2 is replaced happens again in Photosystem 1. The only difference between what happens in Photosystem 2 and Photosystem 1 is the name of the special pair, just P680 or P700, and where the electron comes from that's going to replace the special pair. So, in Photosystem 2, that's going to come from splitting water, and in Photosystem 1, that's going to come from the Photosystem 2 electron.

So, then, let me come back, then we have step 8, so now we have ferredoxin, which is carrying this electron from Photosystem 1, and it carries the electron to what's known as NAD+ reductase. And what this does is it forms NADPH in the stroma, 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 NAD+, plus reductase and that ends up producing NADPH plus hydrogen protons. And then finally, in step 9, 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 ATP synthase and forms ATP.

So, if we are just going to do an overview of photosynthesis, what we see is that this is the thylakoid lumen, so this is the thylakoid membrane. So what happens is we first, the light comes into Photosystem 2. That energy is going to jump around into different things, but eventually it's going to reach a special pair called P680 that's going to donate an electron and it's going to travel to plastoquinone. That electron is replaced by water. Now plastoquinone transfers this to the cytochrome b 6f complex, which uses that energy to pump hydrogens, then that unexcited electron now goes to plastocyanin. From here, light comes into Photosystem 1. It jumps around, jumps around, the energy jumps around until eventually it excites an electron in the P700. Now the P700 is then going to donate an electron, which eventually goes to ferredoxin, and that's going to be replaced by the electron carried by plastocyanin. Now that ferredoxin has this electron, it is going to go through the NAD+ reductase, which is going to form NADPH, and, this whole time we have been making hydrogen, gradients, and this hydrogen, it creates this electrochemical gradient here, which in step 9 is going to flow through ATP synthase to create ATP. So, it is a lot, but, it 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 photosystems that lead to the production of NADPH and ATP. So that's the overview of there those are the nitty-gritty of the light dependent reactions. So with that, let's now move on.

Cyclic Light Dependent Reactions

Okay. So in this video, we're going to talk about the cyclic light-dependent reactions. So previously, when we were 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 cyclic light-dependent reactions, there is this cycle where it circles back and it does something different. So the cyclic light-dependent reactions, that process creates ATP, but not NADPH. And so the reason it does this is that photosystem 2 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 photosystem 1, which instead of transferring its electron to, what is known as NADP+ reductase, it instead goes back to the cytochrome complex. So let me show you this image while I talk about it. So this is photosystem 1, so this is going to be the second half of the light-dependent reactions. So this light comes in, it jumps around, it eventually ends in the special pair P700, and that donates an electron to the primary electron acceptor, and we get to doxin. Now, normally, in the linear pathway, it's going to go through NADP+ reductase. 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, it instead cycles back to before and goes to the cytochrome complex. And this ends up in the production of ATP, 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 ATP instead of NADPH, and then it cycles back through, and replaces itself. And so, yeah, so that is the cyclic reactions, which are different and used much more rarely. Like, this is if the cell needs more ATP, but not NADPH, 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, so I need to tell you about it. So that's the cyclic pathway. Let's now move on.

Which part of the photosystem is responsible for accepting a light photon?

• A. Chlorophyll
• B. Reaction Center
• C. Primary Acceptor
• D. Cytochrome

Oxygen is formed by a reaction occurring where?

• A. Photosystem I
• B. Photosystem II
• C. Cytochrome B6F complex
• D. NADP+ reductase

Where in the chloroplast is NADPH synthesized?

• A. Thylakoid membrane
• B. Grana
• C. Thylaoid lumen
• D. Stroma

Cyclic photophosphorylation is different than photosynthesis in what way?

• A. Photosystem II reverses and generates CO₂
• B. Photosystem I reverses and transports and electron to the electron transport center
• C. The entire process generates more NADPH than ATP
• D. Photolysis doesn't occur

Here’s what students ask on this topic:

What are the light-dependent reactions in photosynthesis?

Light-dependent reactions are the initial phase of photosynthesis, occurring in the thylakoid membrane of chloroplasts. These reactions involve two photosystems: Photosystem II (PSII) and Photosystem I (PSI). Light energy excites electrons in chlorophyll, initiating a series of energy transfers. In PSII, the excited electron is transferred to plastoquinone, which moves through the electron transport system, creating a proton gradient used by ATP synthase to produce ATP. In PSI, light absorption leads to the transfer of electrons to ferredoxin, resulting in the formation of NADPH. These reactions convert light energy into chemical energy in the form of ATP and NADPH, which are used in the Calvin cycle.

What is the role of chlorophyll in light-dependent reactions?

Chlorophyll plays a crucial role in light-dependent reactions by absorbing light energy. It contains a unique chemical structure with a light-absorbing ring called a porphyrin ring, which has easily excitable electrons. When chlorophyll absorbs light, its electrons become excited and gain energy. This excited state allows the electrons to be transferred to other molecules, initiating the process of converting light energy into electrical and then chemical energy. Chlorophyll is found in the light-harvesting complexes of photosystems, where it facilitates the initial steps of photoexcitation and energy transfer.

How do Photosystem II and Photosystem I differ in their functions?

Photosystem II (PSII) and Photosystem I (PSI) have distinct roles in light-dependent reactions. PSII is the first step, where light energy excites electrons in chlorophyll, leading to the splitting of water molecules (photolysis) and the release of oxygen. The excited electrons are transferred to plastoquinone and then through the electron transport system, creating a proton gradient used to produce ATP. PSI, on the other hand, absorbs light energy to re-excite electrons received from PSII. These electrons are transferred to ferredoxin and then to NADP⁺ reductase, forming NADPH. Thus, PSII primarily generates ATP, while PSI produces NADPH.

What is the cyclic electron flow in light-dependent reactions?

Cyclic electron flow is an alternative pathway in light-dependent reactions that generates ATP without producing NADPH. In this process, electrons from Photosystem I (PSI) are excited by light and transferred to the primary electron acceptor, ferredoxin. Instead of moving to NADP⁺ reductase, these electrons cycle back to the cytochrome b₆f complex, which pumps protons into the thylakoid lumen, creating a proton gradient. This gradient drives ATP synthesis via ATP synthase. The cyclic electron flow is used when the cell requires more ATP but not additional NADPH, balancing the energy needs of the cell.

What is the significance of the proton gradient in light-dependent reactions?

The proton gradient is essential in light-dependent reactions as it drives the synthesis of ATP. During these reactions, the energy from excited electrons is used by the cytochrome b₆f complex to pump protons (H⁺) from the stroma into the thylakoid lumen, creating an electrochemical gradient. This gradient represents stored energy, as there is a higher concentration of protons inside the thylakoid lumen compared to the stroma. ATP synthase, an enzyme embedded in the thylakoid membrane, utilizes this gradient to convert ADP and inorganic phosphate (P_i) into ATP, providing the cell with usable chemical energy.