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Conceptual overview of light dependent reactions

During the light-dependent reactions of photosynthesis, light energy excites electrons, which then move through a series of molecules in the thylakoid membrane of chloroplasts. As the electrons move to lower energy states, they help pump hydrogen ions into the thylakoid lumen, creating a concentration gradient. This gradient powers ATP synthase to produce ATP.

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Video transcript

- [Voiceover] We've seen in previous videos that photosynthesis can be broken down into the light-dependent reactions and the Calvin cycle. And the light-dependent reactions is where we take light as an input along with water, and we'll see the water is actually a source of electrons, and we can use that to store energy in the form of ATP and NADPH, and as a by-product we produce molecular oxygen, which is very important for us to breathe. And then that ATP and that NADPH can be used in the Calvin cycle, along with carbon dioxide, to actually synthesize sugar. What we're gonna focus on in this video are the light-dependent reactions. How does this process right here work? And to help us think about this, we're going to zoom in onto a thylakoid membrane. So this is a thylakoid right over here, sitting inside of the chloroplast. And if we zoom in on its membrane, we see it's a phospholipid bilayer, like many membranes that we see in biology. And at first glance, this might seem like a very complex diagram, and that's because it is a complex diagram, and you will often see things like this in your biology textbooks. They can be very intimidating, these proteins and molecules and complexes have very complicated-sounding names, but the general idea of what's going on is, you'll hopefully find, pretty straightforward. You have the energy from light, photons from light are going to, either directly or indirectly, excite electrons. Those excited electrons, they're in a high-energy state. They're gonna be transferred from one molecule to another, and they're going to go to lower energy states. That's what allows those transfers to be as spontaneous, for them to actually occur. They're going from a high-energy state to a lower-energy state. The electrons are getting more and more and more comfortable, and some of that energy that's released as the electron goes from a high-energy state to a lower-energy state is used to pump hydrogen ions across the membrane. From the outside of the membrane, in the stroma, to the inside of the membrane, to within the thylakoid lumen. So you are building a hydrogen ion concentration gradient. Concentration gradient. Where you have a higher concentration inside than you have outside. And this by itself, this concentration gradient as we'll see, can be used to fuel the production of ATP by ATP synthase, that those hydrogen ions want to get back out. They wanna go down their concentration gradient, and as they go back out through the ATP synthase it essentially turns that motor that can jam the phosphate group onto ADP to produce ATP. So one way to think about it, this is producing a hydrogen ion gradient. So we could do it this way, we could say H plus gradient, which is then being used to produce the actual ATP. Now the electrons going from a high-energy state to a lower-energy state in this part of the light-dependent reactions, that by itself isn't the only thing that is contributing to the hydrogen ion concentration gradient. Once that electron gets donated, you might say, well how does it get replaced? Well the thing that's doing the donating, the thing that eventually gets excited and donates that electron, it's a chlorophyll a variation called P680. P680 is referring to the P stands for pigment, 680 stands for 680 nano-meters, the wavelength of light that it absorbs best. And so when it gets excited, it becomes you'll see the notation off of P680*, that's when it has an excited electron. And then after it gives away its electron, it becomes P680+ with a positive charge. And this P680, we could call it P680 plus right over here, maybe a P680 ion, this is actually a very strong oxidizing agent. One of the strongest, if not the strongest, that we know in biological systems. And so it really likes to grab electrons from other things. And the thing that is around that it can grab electrons from is actually water. And so this is such a strong oxidizing agent that it can essentially oxidize the oxygen in water, and oxygen as itself. I mean, oxidizing is named after oxygen because oxygen is such a strong, it's so electronegative, it's such a strong, it's the thing that's normally doing the oxidizing. So anyway, it grabs its electrons, once it gets this P680+, grabs an electron from water, and then the water essentially falls apart so you're left just with the oxygen and then the hydrogen ions. And so those hydrogen ions also contribute to the increased hydrogen ion concentration on the inside. And this is where we get the oxygen by-product right over here. Here we have one half of an O2, so if you do this twice you're going to have a molecular oxygen. So, so far we've talked about how the oxygen gets produced, we've talked about how the ATP gets produced. What about the NADPH? Well we've started our process in photosystem II. You might say, why's it called photosystem II if that's where we start? Well it's actually that's because that's the second photosystem to be discovered. You might say, what is a photosystem? Well these photosystems and complexes, they're combinations of proteins and molecules, and photosystem in particular has chlorophyll and variations of chlorophyll and pigment molecules that are responsive to light that are very easy, that have electrons that can get excited by light, and they can transfer that energy back down to the P680 chlorophyll a pair, which then can have its electron excited and then it can give that to an acceptor molecule and then it can go to lower, lower energy states and pump those hydrogen ions out. But that's not the entire light-dependent reactions. That electron can eventually make its way over to photosystem I, and why's it called photosystem I? Well it's because the first one that was discovered. In photosystem I, there's another chlorophyll a pair called P700, and that's because it optimally absorbs light of a wavelength of 700 nano-meters. And you have something similar that happens, that light can either directly or indirectly excite its electron. And then that electron, as it goes to a lower-energy level, it goes from one molecule to another, it can be used to reduce NADP+ into NADPH. And so that's where the NADPH comes from. And then once again, once the P700 has given its electron, it wants an electron, and well it can get that from the electron that's been going from lower to lower, lower energy states, that's essentially been making its way from, you can conceptualize it as the electron that's been making its way from photosystem II. And so that's why you'll often see these diagrams. Lights come in, electron gets energized, it gets excited, it goes to lower and lower energy states. As it's doing that it's being transferred from one molecule to another, being facilitated by enzymes. That energy, part of that energy is being used to transfer hydrogen ions into the thylakoid lumen, into the interior. Then in photosystem I, you have another excitation event. That thing that got excited can grab that electron that went to lower, lower energy states, and its excited electron can once again be transferred from one molecule to another in order to fuel or provide energy for NADP+ being converted into NADPH. And once again the whole idea of the hydrogen ion concentration increasing here can fuel ATP synthase, which allows us to jam a phosphate onto ADP to produce ATP. So that is where we actually get all of these things and the by-product of course is our oxygen. And if you wanted to see that same idea but kind of just thinking from an energetic point of view without all of the complexity of seeing the physical components involved, you see it right over here. Where you have light energy comes, excites the electrons. Once the P680 has given that electron away, it wants an electron really badly. It gets it from the water. And then as that electron goes to lower and lower and lower energy states, it can eventually be grabbed by P700 that has given away its own electron. And then that electron that was excited at P700 by, once again, more light energy, that can be transferred from one molecule to another to fuel the creation of NADPH. And this part right over here, this phase right over here, as the energy goes from a high-energy state to a- as the electron goes from a high-energy state to a lower energy state, fuels the pumping of hydrogen protons into the actual thylakoid.