The Most Important Process on Earth

the second law of thermodynamics is a cruel mistress its existence means everything and everyone is always moving down a proverbial hill we can of course move back up hills but only by pushing other things down their own we are constantly pushed up hills by the energy stored in the bonds between carbon hydrogen and oxygen as these atoms roll down their own hills we can use that energy to keep us alive and moving this is the principle of cellular respiration unfortunately the resulting destinations for these atoms water and carbon dioxide are the bottoms of their own respective hills that means these molecules are worthless for basically any biological reaction we can of course push carbon hydrogen and oxygen back up their hills but this requires pushing something else downhill and frankly our supply of things to push down hills is limited here on earth but within all nooks crannies and spaces large and atomic in scale the electromagnetic field is constantly buzzing and vibrating with energy and potential that we call photons if only we could reach out and grab these vibrations we could use that energy to push our fueling atoms back up their hills without having to push anything else here on earth down their own something like that just might be the most important process on earth photons are excitations in the electromagnetic field and directly interact with electrons i recommend my video on blackbody radiation to understand this interaction further if we could find a way to take the energy from a photon and pass it into an electron suddenly we have a form of electricity or power we can harness for a useful process this is the principle of photosynthesis plants use the energy stored within the electromagnetic field to drive crucial cellular processes photosynthesis can be quite a complicated thing so we'll go step by step to unravel what's happening along the way the first question we can ask is how do plants capture sunlight this is accomplished by synthesizing molecules called pigments within the chloroplasts of plant cells although all matter interacts with light pigments are molecules who interact strongly with visible light so our next question is why and how are pigments so good at absorbing light there are multiple different pigments each structured to absorb a different wavelength of light you can think of them as antennas tuned to specific radio stations but instead of frequencies we use wavelengths the main pigment in all eukaryotic photosynthetic organisms is chlorophyll alpha normally electrons can only absorb photons in very discrete amounts they're constrained by the orbitals and energy states within the molecules they make up but this alternating double and single bond pattern throughout chlorophyll's ring creates a conjugated system which is a fancy way of saying there are more energy states than meets the eye you can rearrange these double bonds in multiple different ways creating multiple different opportunities for an electron to absorb a photon add in the magnesium atom and its influence on the energy states in each bond and you are ripe for photon interaction this is what makes pigments special they all have this property of a conjugated system which allows them to absorb a wider spectrum of light now we ask what does a pigment do when it absorbs light pigments can be found embedded in specialized membranes of what we call antenna complexes antenna complexes are connected to a reaction center and these two components make up what we call a photosystem photosystems are themselves embedded in the membrane of thylakoids we'll learn about those later but it's important to orient ourselves to break down what happens after a pigment absorbs a photon when a pigment or electron absorbs a photon it jumps into an excited state while excited an electron is very unstable and can do three things the first is that it can simply leave the molecule if there's another molecule nearby looking to accept it if it can't leave then its instability causes it to return back to its ground state in this instance it will either release its excess energy in the form of a photon or if the molecule is close to another molecule with a similar electron it can pass that energy in a quantum event called resonance energy transfer this passive energy is instantaneous and undetectable no photons are exchanged rather it is a quantum interaction of the nearby electron wave functions is in this manner that energy absorbed from a pigment is passed around at random with nearby pigments in the antenna complexes of a photosystem eventually that excitation will be transferred to the reaction center in every reaction center are two chlorophyll alpha molecules when the energy is then passed on to them the excited electron is suddenly in close proximity to an electron acceptor molecule called phaophytin this is a chlorophyll molecule but instead of a magnesium it has two protons in its center after an electron is passed to the phyophyton it is then passed to plastic keynote a which after collecting two electrons passes these to plastic keynote b who then grabs two protons from the stroma or outside area of the thylakoid finally plastic keynote b passes the two electrons to a cytochrome protein complex and dumps the resulting protons into the lumen or interior of the thylakoid what i've described is the process of photosystem ii it serves principally to bring protons from outside of the thylakoid into the interior as this concentration of protons builds up they are funneled through atp synthase as they pass through this protein it spins and this kinetic motion is used to synthesize atp the second photosystem photosystem 1 uses the electrons freed from its reaction center to reduce nadp positive to nadph photosystem 1 however relies on photosystem 2. remember the cytochrome protein complex that freed up the electrons from plastic keynote b well those electrons need to go somewhere and that just so happens to be the reaction center of photosystem 1. now the electrons from photosystem 1 get passed down a chain to wind up with ferrodoxin which is then used to reduce nadp positive in a large protein complex aptly named ferrodoxin nadp positive reductase those are the two roles of the two photosystems but we are missing two more key processes in photosynthesis the first is something you've hopefully questioned how does photosystem 2's reaction sensor replenish its electrons this is accomplished by splitting water into oxygen and more protons for our proton pumps one of these little lumps of proteins in photosystem 2 is yet another protein complex called the oxygen evolving complex this complex stores and uses the energy of four photons to split two water molecules using manganese atoms in this process of splitting two water molecules it creates four protons one oxygen molecule and four electrons these electrons are passed via a tyrosine residue to the reaction center thus allowing the next photon energy to potentially trigger another electron release from the reaction center the two photosystems work in unison to facilitate the real magic of photosynthesis that being the uphill reaction of converting carbon dioxide into glucose and sucrose this is the infamous calvin cycle and it's a bit complex when read about but i think visualizing it will make it a bit easier to understand the calvin cycle is broken into three steps fixation reduction and acceptor regeneration to begin we have three molecules of rabulos biphosphate these are independent of our light reactions three carbon dioxide molecules enter and are enzymatically bound by the infamous rubisco protein to each ribulose molecule this creates an unstable intermediary which immediately collapses forming molecules of phosphoglycerate each one of these molecules reacts with atp to create biphosphoglycerate in reduction each biphosphoglycerate is reduced by nadph into glyceraldehyde-3-phosphate in the final step acceptor regeneration we need to create the original regular sugars that started this cycle we will soon discover we only need five of our six phosphoglyceraldehydes to reform our initial ribulose molecules so our extra sixth goes off to make bigger sugars this step was a bit strange to me because no one anywhere wants to explain it everywhere i looked the last step was simply then the five phosphoglyceraldehydes reform into three ribulose molecules via atp but that doesn't make sense you can't just combine these together if you've taken organic chemistry you know phosphates generally attack the carbonyl carbon so that can't be the first step after a ton of searching i found this proposal from queen mary in london i'm not going to translate this but you can see it's yet another complex reaction with its own intermediaries i don't know why this is excluded from textbooks i understand it would probably be too much to learn on top of the calvin cycle but i think it's important to marvel at how wild and complex life and its processes are and that's how photosynthesis undergoes the most important process on earth photosystems capture and convert light energy into chemical energy photosystem 2 moves protons into the thylakoids which creates atp as it leaves this process indirectly creates oxygen photosystem 1 frees electrons that are used to reduce nadp positive into nadph atp and nadph are then used as sources of energy to convert carbon dioxide into sugars and what we call the calvin cycle so the next time you're using any source of energy be that from your food the wind or dirty coal at a power plant marvel at the novelty that you can trace that energy all the way back to the fusion of atoms in the center of a star [Music] you

Channel: But Why?

Views: 43,641

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Keywords: photosynthesis, photosystem, photosystem I, photosystem II, Calvin, cycle, ATP, synthase, thermodynamics, botany

Id: or5qN0SHJC4

Length: 11min 11sec (671 seconds)

Published: Thu Apr 15 2021