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Video transcript

photosynthesis it is not some kind of abstract scientific thing you would be dead without plants and their magical nay scientific ability to convert sunlight carbon dioxide and water into glucose and pure delicious oxygen this happens exclusively through photosynthesis a process that was developed 450 million years ago and actually rather sucks it's complicated inefficient and confusing but you are committed to having a better deeper understanding of our world or more probably you'd like to do well on your tests so let's delve there are two sorts of reactions in photosynthesis the light dependent reactions and the light independent reactions and you've probably already figured out the difference between those two so that's nice the light independent reactions are called the Calvin cycle no not no no no yes that Calvin cycle photosynthesis is basically respiration in Reverse and we've already covered respiration so maybe you should just go watch that video backwards or you could keep watching this one either way I've already talked about what photosynthesis needs in order to work water carbon dioxide and sunlight so how do they get those things first water let's assume that we're talking about a vascular plant here that's the kind of plant that has pipe like tissues that conduct water minerals and other materials to different parts of the plant these are like trees and grasses and flowering plants in this case the roots of the plants absorb the water and bring it to the leaves through tissues called xylem carbon dioxide gets in and oxygen gets out through tiny pores in the leaves called stomata it's actually surprisingly important that plants keep oxygen levels low inside of their leaves for reasons that we will get into later and finally individual photons from the Sun are absorbed in the plant by a pigment called chlorophyll all right you remember plant cells if not you could go watch the video where we spend the whole time talking about plant cells one thing the plant cells have that animal cells don't plastids and what is the most important plastic the chloroplast which is not as it is sometimes portrayed just a big fat sack of chlorophyll it's got complicated internal structure now the chlorophyll is stashed and membranous sacs called thylakoids and the thylakoids are stacked into grana inside the thylakoid is the lumen and outside of the thylakoid but still inside of the chloroplast is the stroma the thylakoid membranes are phospholipid bilayers which if you remember means that they are really good at maintaining concentration gradients of ions and proteins and other things this means keeping a concentration higher on one side than the other of the membrane you're going to need to know all of these things I'm sorry now that we've taken our little tour of the chloroplast it's time to get down to the actual chemistry first thing that happens a photon created by the fusion reactions of our Sun is about to end it's 93 million mile journey by slapping into a molecule of chlorophyll this kicks off stage 1 the light dependent reaction is proving that yes nearly all life on our planet is fusion power chlorophyll gets hit by that photon on electron absorbs that energy and gets excited this is the technical term for electrons gaining energy and not having anywhere to put it and when it's done by a photon it's called photo excitation but let's just imagine for the moment anyway that every photon is whatever dreamy young man twelve-year-old girls are currently obsessed with and electrons are twelve-year-old girls the trick now and the entire trick of photosynthesis to convert the energy of those 12 euro I mean electrons into something that the plant can use we are literally going to be spending the entire rest of the video talking about that I hope that that's okay with you that first chlorophyll is not on its own here it's part of an insanely complicated complex of proteins and lipids and other molecules called photosystem ii that contains at least 99 different chemicals including over 30 individual chlorophyll molecules this is the first of four protein complexes that plants need for the light dependent reactions and if you think it's complicated that we call the first complex photosystem 2 instead of photosystem 1 then you're welcome to call it by its full name which is plasto quinone oXXO reductase oh no you don't want to call it that right then photosystem 2 or if you want to be brief ps2 ps2 and indeed all of the protein complexes and the light dependent reactions straddle the membrane of the thylakoids in the chloroplasts now that excited electron is going to go on a journey designed to extract all of its new energy and convert that energy into useful stuff this is called the electron transport chain in which energized electrons lose their energy in the series of reactions that capture the energy necessary to keep life living so PS 2's chlorophyll now has this electron that is so excited that when a special protein designed specifically for stealing electrons shows up the electron actually leaps off of the chlorophyll molecule onto the protein which we call a mobile electron carrier because it's a mobile electron carrier chlorophyll then freaks out like a mother who has just had her 12 year old daughter abducted by a teen idol and I was like what do I do to fix this problem and then it in cooperation with the rest of photosystem 2 does something so amazing and important that I can barely believe that it keeps happening every day it splits that ultra-stable molecule h2o stealing one of its electrons to replenish the one it lost the byproducts of this water splitting hydrogen ions which are just single protons and oxygen sweet sweet oxygen this reaction my friends is the reason that we can breathe brief interjection next time someone says that they don't like it when there are chemicals in their food please remind them that all life is made of chemicals and would they please stop pretending the word chemical is somehow a synonym for carcinogen because I mean think about how chlorophyll feels when you say that it spends all of its time and energy creating the air we breathe and we're like air chemicals are so gross now remember all energized electrons from PS 2 have been picked up by electron carriers and now are being transported to the second protein complex the cytochrome complexes little guy does two things one it serves as an intermediary between PS 2 and PS 1 and 2 uses a little bit of that energy from the electron to pump another proton into the thylakoid so the final Accord starting to fill up with protons we've created some by splitting water and we moved one and using the cytochrome complex but why are we doing this well basically what we're doing is charging the thylakoid like a battery by pumping the thylakoid full of protons we're creating a concentration gradient the protons then naturally want to get the heck away from each other and so they push their way through an enzyme straddling the thylakoid membrane called ATP synthase and that enzyme uses that energy to pack an inorganic phosphate onto ADP making ATP the Big Daddy of cellular energy all of this moving along the electron transport chain requires energy and as you might expect electrons are entering lower and lower energy states as we move along this makes sense when you think about it it's been a long while since those photons zapped us and we've been pumping hydrogen ions to create ATP it's putting water and jumping onto different molecules tired just talking about it luckily as 450 million years of evolution would have it our electron is now about to get re-energized upon delivery to photosystem 1 so PS 1 is a similar mix of proteins and chlorophyll molecules that we saw on ps2 but with some different products after a couple of photons we excite a couple of electrons the electrons pop and hitch a ride onto another electron carrot this time all of that energy will be used to help make NADPH which like ATP exists solely to carry energy around here yet another enzyme helps combine two electrons and one hydrogen ion with a little something called nadp+ as you may recall from our recent talk about respiration there are these sort of distant cousins of B vitamins that are crucial to energy conversion in photosynthesis it's nadp+ and when it takes on those two electrons and one hydrogen ion it becomes NADPH so what we're left with now after the light-dependent reactions is chemical energy in the form of ATP s and NADPH s and also of course we should not forget the most useful useless byproduct in the history of useless byproducts oxygen if anybody needs a potty break now would be a good time or if you want to go rewatch that rather long and complicated bit about light dependent reactions go ahead and do that it's not simple and it's not going to get any simpler from here because now we are moving along to the Calvin cycle the Calvin cycle is sometimes called the dark reactions which is kind of a misnomer because they generally don't occur in the dark that occur in the day along with the rest of the reactions but they don't require energy from photons so it's more proper to say light independent reactions or if you're feeling non-descriptive just say stage two stage two is all about using the energy from those ATP's and NADPH s that we created in stage one to produce something that's actually useful for the plant the Calvin cycle begins in the stroma or the empty space inside of the chloroplast if you remember correctly and this phase is called carbon fixation because yeah we're about to fix a co2 molecule onto our starting point ribulose bisphosphate or rubp which is always around in the chloroplast because not only is it the starting point of the calvin cycle it's also the end point which is why it's a cycle co2 is fixed to ru BP with the help of an enzyme called ribulose 1/5 bisphosphate carboxylase oxidase which we generally shortened to rebus Koh I'm in the chair again excellent this time for a biography of Rubisco once upon a time a one-celled organism was like man I need more carbon so I can make more little Me's so I can take over the whole world luckily for that little organism there was a lot of co2 in the atmosphere and so it evolved an enzyme that could suck up that co2 and convert inorganic carbon into organic carbon this enzyme was called Rubisco and it wasn't particularly good at its job but it was a heck of a lot better than just hoping to run into some chemically formed organic carbon so the organism just made a ton of it to make up for how bad it was not only did the little plains stick with it it took over the entire planet rapidly becoming the dominant form of life slowly through other reactions known as the light dependent reactions plants increased the amount of oxygen in the atmosphere Rubisco having been designed in a world with tiny amounts of oxygen in the atmosphere started getting confused as often as half of the time Rubisco started slicing ribulose bisphosphate with oxygen instead of co2 creating a toxic by-product that plants had to deal with and creative in specialized ways this by-product called phosphoglycerates believed to tinker with some enzyme functions including some involved in the calvin cycle so plants have to make other enzymes that break it down into an amino acid glycine and some compounds that are actually useful to the Calvin cycle but plants had already sort of gone all-in on the Rubisco strategy and to this day they have to produce huge amounts of it scientists estimate that at any given time there are about forty billion tonnes of Rubisco on the planet and plants just deal with that toxic byproduct another example my friends of unintelligent design back to the cycle so ribulose bisphosphate get to co2 slammed onto it and then immediately the whole thing gets crazy unstable the only way to regain stability is for this new six carbon chain to break apart creating two molecules of 3-phosphoglycerate and these are the first stable products of the Calvin cycle for reasons that will become clear in a moment we're actually going to do this to three molecules of rubp now we enter the second phase reduction here we need some energy so some ATP slams a phosphate group onto the 3 phosphoglycerate and then NADPH pop some electrons on and voila we have two molecules of glyceraldehyde 3 phosphate or g3p this is a high-energy 3 carbon compound that plants can convert into pretty much any carbohydrate like glucose for short-term energy storage cellulose for structure starts for long-term storage and because of this g3p is considered the ultimate product of photosynthesis however unfortunately this is not the end we need five g3ps to regenerate the three rubp s that we started with we also need nine molecules of ATP and six molecules of NADPH so with all these chemical reactions all of this chemical energy we can convert three rubp s into six g3ps but only one of those g3ps gets to leave the cycle the other g3ps of course being needed to regenerate the original three ribulose bisphosphate that regeneration is the last phase of the calvin cycle and that is how plants turn sunlight water and carbon dioxide into every living thing you've ever talked to played with climbed on loved hated or eaten not bad plants
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