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Review 4: Amino Acid Oxidation, Oxidative Phosphorylation, & Photophosphorylation

Photophosphorylation 2


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photosynthesis occurs in chloroplasts, and these are organelles that have electron transport chains and ATP synthesis and convoluted inner structures. Very similar. Thio mitochondria. Um, this is also the organ L in which the Calvin cycle occurs where carbon dioxide is fixated into carbohydrates. But we're not going to get into any of that now. We should talk a little bit about chloroplast anatomy. The goopy fluid that fills Corp Plast is called the S Troma and these structures inside the chloroplast these in fold ings are called File a koi AIDS and Thilo coins. As you can see over here, have a bloomin inside. So there is a new internal space, and that is where our proton concentration is going to build up. But more on that momentarily. Now the reactions, uh, that we're going to be covering are called the light dependent reactions. And that's because they require light energy to perceive the Calvin cycle, sometimes called the light independent reactions, because they don't actually need sunlight energy to go. Sometimes they're also called the dark reactions. But that's kind of misleading because they do occur in light as well. But anyways, we're going to focus on light reactions and in light reactions. What's going to happen is we're going toe. We're going to be producing a teepee, right? That's one of the main things we're also going to be producing. N a. D pH And remember, n a D. PH is very similar to N A. D H Solectron carrier and N A. D. PH is actually what's used in the Calvin Cycle. Thio in part Thio, uh, generate thes carbohydrates, but we're not going to get into that. So that gets used up in the Calvin cycle and comes back as any d p plus to the light reactions. The the light reactions will actually see split water and take electrons from it and oxidize the oxygen's thio molecular oxygen, which will be released from these reactions. And that is the oxygen that is released from plants that we breathe. Pretty cool, huh? All right. So you can see, uh, the equation for the light reactions right here you have to h 202 and a d p plus and you form That's should be two and a D. P. H. Two h plus and + Now the photo systems that we're going to be talking about are basically complexes of proteins and photo pigments and other organic molecules embedded in the Fila coid membrane. And photo systems can basically be broken down into two parts. You have your light harvesting complex, which is this outer portion, and then you have the reaction center, which is this interior portion. So this whole thing here is a photo system. Now the light harvesting complex is basically an antenna. It's chock full of tons of chlorophyll, carotenoids and other photo pigments, and its job is to pick up light energy. It wants to get light energy, and as soon as it gets it, it wants to transfer it and wants to basically move it through the photo system. I'm sorry, through the light harvesting complex to the reaction center, and the way this occurs is very interesting thing. It's basically, uh, on energy transfer, a very special type of energy transfer called Resonance Energy transfer. And essentially, what happens is a new electron in one photo, pigment gets excited, right by light photon hits it. It bounces up into this excited state, and when it relaxes, it actually excites its neighbor, so its neighbor gets excited and then that electron relaxes and excites its neighbor. And that's how this energy is relayed around the light harvesting complex until it is transferred to the reaction center, too thes special pigments in the reaction center. Now the reaction center contains Chlorophylls, cida, Chrome's Quinones and these things called fio fighting, which are basically chlorophylls without magnesium. They're all electron carriers, as I'm sure you can assess, I'm sure, you know. And essentially, once the energy from the light harvesting complex hits the reaction center, it will excite on electron in the Reaction Center, exciting electron from chlorophyll in the reaction center so much that it actually gets ejected. So I almost like to think of the photo system like, you know, micro magnifying glass. If you ever have ever seen a magnifying glass, focus. Sunlight, energy right. Focus. Sunlight, energy, and then you can burn something with it. That's kind of what's going on here with light harvesting Complex focuses the energy onto the reaction center, and that allows the Reaction Center to excite its electron enough to actually eject it. Not just excited, but eject it so that it gets picked up by the electron carriers and taken around in a manner that's very similar to the electron transport we just saw. I mean, it is it is an electron transport chain. It's just a different one than the one we were just looking at in the mitochondria. Now the electrons can actually come back. Thio come back to the reaction center so mhm they will, uh the energy will get passed around, right. You'll excite that electron, and it will move through electron transport chain into this cytochrome complex. And we'll talk all about this and more detail momentarily. And moving through the cytochrome complex is actually what causes that protein complex to pump protons and create a proton. Radiant. Now, the, uh, this process will. As we'll see, we'll play out in a fashion where the electrons can be delivered to the reaction center of another photo system. Right? So once a photo system loses its electrons that needs them replaced so they could be delivered to the reaction center of another photo system and ultimately be given to N a. D p plus reductase to turn n a d p plus into N a. D. PH. Right. And then that's gonna go off to the Calvin cycles. That's one thing that can happen. But what can also happen is instead of delivering them to N a. D P reductase, do that or this molecule or this protein. Rather, Farid oxen can actually shuttle the electrons back to the cytochrome complex in a cyclic fashion. So essentially electrons can be used just to create the proton radiant. That's gonna ultimately power 80 p synthesis or the electrons can be used to reduce N a d p plus two u. N A. D. PH and I have the P in parentheses here because in some circumstances it will be n a D plus and others n a d p plus for various photosynthetic organisms. For plants, it's N a d p on a d p plus. Anyhow, let's flip the page and take a look at this process in more detail.