Hello everyone. Welcome back to my Coursera class which is about Biochemical Principles of Energy Metabolism. So this is the second session of week four which is a Photosynthesis lecture. So let's begin with the, again, the chemical formula for photosynthesis, carbon dioxide and water. And finally, we have to produce glucose, I mean do we mean plant cells and then oxygen to be released. And then, as I said, this reaction is really, really endergonic and very difficult to occur without additional energy source from the sun, I mean the photons. Let me give you the overview of photosynthesis. As I said, in the previous, the first session of week four, what photosynthesis is divided into two reactions, light-dependent pathway and the Calvin cycle. This is light-independent, light-independent. So today, we are going to talk about this one, light-dependent reactions. Light-dependent reactions, light coming in and that energy is utilized to drive ATP synthesis, and to produce the electron carriers NADPH and oxygen released. In the very beginning, the light energy, they are supposed to be absorbed. And then we need and plant cells require specialized pigments. The most famous one pigment is chlorophyll A and B. A and B both are very, very good at absorbing the red light and the deep blue light. That's why most plant cells show us more green color. Because green colors cannot be absorbed and rather to reflect it. And then what's going on after the light energy absorption? The thing is this one, when photons arrive at the chlorophyll, the pigment, the conjugated system, the photons indeed excite electrons. And excited electrons, changing to from the ground state into higher electron orbitals, which means excited to the state. And then we have to talk about these specialized complex, light-harvesting complex, which is localized chloroplast, outer membrane, inner membrane, thylakoid membrane contains these light-harvesting complex. So this is a complex of proteins and chlorophyll pigments, and they are localized and embedded inside the thylakoid membrane. So when the solar energy, I mean photons arrive and get absorbed. So when you see these structures, these helical rot indicate proteins and those purple stick and balls and those are chlorophylls. Chlorophylls are heavily embedded and they absorb all light energy. And then, as I explained in the previous slide, once the energy from photons delivered to the target electrons that those electrons are excited. So in particular, we have to think about the fate of electron. Very interestingly, light-harvesting complex, in particular, the photosystem. I'm going to show this photosystem, the diagram in more detail later, that there is a protein and chlorophyll complex which is responsible for absorbing light, in particular, the photosystem chlorophyll reaction center. Once electron is excited throughout those photon-induced energy absorption, and then that electron, once activated, it can be transferred to an adjacent neighbor of molecule. So in that case, when this electron is transferred physically, moved to adjacent acceptor molecules, these donor molecules lose one electron. That's why these donor becomes positively charged. There is electron deficit loss, and then acceptor chlorophyll or other molecules have additional electrons. That's why this is negatively charged, obtained additional electrons. So the point is once the huge amount of light energy absorbed throughout those photosystem and specialized chlorophyll pairs, the electron is indeed a physically removed from specialized donor chlorophyll, and that electron is traveling through downstream electron transport system. Let me show the summarized cartoon for light-induced photosynthesis, light-dependent reactions. When sunlight comes in, and then specialized chlorophyll are activated, in particular, the activated electron physically detached from the donor chlorophyll pigments. And that electron is leaving to downstream electron carriers and acceptors. And that electron indeed, going through electron transport system. This electron transport system found in chloroplast are very, very similar and, in functionally, analogous to Mitochondria electron transport system. These high energy containing. That high energy ultimately comes from the sun. Electron transport system throughout this electron transfer reactions, protons can be translocated, and then proton concentration can be increased inside thylakoid structure. I mean this space of thylakoid, inner space of thylakoid membrane, luminal compartment. And then electrons, finally, destined into NADP+ and finally, reduced NADPH level can be increased. So biochemical principal is very very clear. Light-dependent, photo excitation electrons from the chlorophyll traveling through the electron transport system. And then throughout those processes, proton gradient can be established. And these proton gradient, so like mitochondria oxydative phosphorylation, can be used to drive ATP synthesis. And on top of this, the plant cells can accumulate reduced NADPH which means electron carriers, electron donors. So light-dependent reactions, the chemical formula is like this. The water, NADP+ oxidized ones are reduced, and then huge amount of ATP can be synthesized. The last piece of the critical information related to light-dependent reaction is the generation of oxygen. Final product ATP, NADPH, energy, electrons and oxygens. So I'm going to show this diagram again. As I explained, electrons removed strato light-dependent photo excitation removed from the specialized chlorophyll pigments and then traveling. The thing is when this electron is removed from specialized chlorophyll inside the reaction center, that electron supposed to be supplied. Supplied from what? That electron donor is water. So photosystem, in particular photosystem two, contains specialized region to oxidize water and abstract electron to compensate for the loss of electrons photoinduced trans separation. And as a by-product, oxygen can be released. And the other by-product is proton. So this is how the photosystem two look like. So in particular, underneath of this membrane embedded approaching chlorophyll complex, there is a specialized region for water oxidation. So four electrons can be stripped from stable water molecules, and then those electrons are continuously supplying for specialized chlorophyll pigment. And oxygens and protons just released away. So in particular, to drive this oxygen generation reaction, to produce one mole of oxygen and two moles of water are supposed to be oxidized, and in this case, four photons required. People analyzed the data, and four heats of photons are enough to produce one oxygen molecule out of water oxidation. So in the real situation, real in vivo living typical chloroplast structure in a plant cell, we can calculate the free energy. So across the thylakoid membrane, proton gradient will be established, right? Throughout those electron transport system. And that Delta pH across this thylakoid membrane is like 3.5. And then we can further calculate the concentration dependent to chemical free energy and electrical free energy. I'm not going to explain everything in detail, and we can finally have this huge amount of free energy captured in this chloroplast proton gradient dependent situation. So I'm going to summarize the light-dependent reaction. So light-dependent reaction is very, very essential in terms of energy production for plant cells and the other major product oxygen as you can clearly understand, the key component of our human life and many lives on this planet. So light-dependent activation of electrons transport system and proton gradient drive ATP synthesis and electrons are captured in a form of NADPH. This is how a plant cells utilize the literally unlimited energy resource from the sun. And how plant cells generate ATP and electron donors NADPH molecules, as well as oxygen as a waste product.
