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photosynthesis! it is not some kind of abstractscientific thing. you would be dead without plants and their magical- nay, scientificability to convert sunlight, carbon dioxide and water into glucose and pure, deliciousoxygen. this happens exclusively through photosynthesis,a process that was developed 450 million years ago and actually rather sucks.

top 10 crash diets that work, it's complicated, inefficient and confusing.but you are committed to having a better, deeper understanding of our world! or, moreprobably, you'd like to do well on your test...so let's delve. there are two sorts of reactions in photosynthesis...lightdependent reactions, and light independent

reactions, and you've probably already figuredout the difference between those two, so that's nice. the light independent reactions arecalled the "calvin cycle" no...no...no...no...yes! that calvin cycle. photosynthesis is basically respiration inreverse, and we've already covered respiration, so maybe you should just go watch that videobackwards. or you can keep watching this one. either way. i've already talked about what photosynthesisneeds in order to work: water, carbon dioxide and sunlight. so, how do they get those things?

first, water. let's assume that we'retalking about a vascular plant here, that's the kind of plant that has pipe-like tissuesthat conduct water, minerals and other materials to different parts of the plant. these are like trees and grasses and floweringplants. in this case the roots of the plants absorbwater and bring it to the leaves through tissuescalled xylem. carbon dioxide gets in and oxygen gets outthrough tiny pores in the leaves called stomata. it's actually surprisingly important thatplants keep oxygen levels low inside of their leaves for reasons that we will get into later.

and finally, individual photons from the sunare absorbed in the plant by a pigment called chlorophyll. alright, you remember plant cells? if not,you can go watch the video where we spend the whole time talking about plant cells. one thing that plant cellshave that animal cells don't... plastids. and what is the most important plastid? the chloroplast! which is not, as it is sometimesportrayed, just a big fat sac of chlorophyl. it's got complicated internal structure. now, the chlorophyll is stashed in membranoussacs called thylakoids. the thykaloids are

stacked into grana. inside ofthe thykaloid is the lumen, and outside the thykaloid (but still inside thechloroplast) is the stroma. the thylakoid membranes are phospholipid bilayers, which, if you remember means they're really good at maintainingconcentration gradients of ions, proteins and other things. this means keepingthe concentration higher on one side than the other of the membrane. you're goingto need to know all of these things, i'm sorry. now that we've taken that little tour ofthe chloroplast, it's time to get down to the actual chemistry. first thing that happens: a photon createdby the fusion reactions of our sun is about

to end its 93 million mile journey by slappinginto a molecule of cholorophyll. this kicks off stage one, the light-dependent reactions proving that, yes, nearly all life on our planet isfusion-powered. when chlorophyll gets hit by that photon,an electron absorbs that energy and gets excited. this is the technical term for electrons gainingenergy and not having anywhere to put it and when it's done by a photon it's calledphotoexcitation, but let's just imagine, for the moment anyway, that every photon iswhatever dreamy young man 12 year old girls are currentlyobsessed with, and electrons are 12 year old girls. the trick now, and the entire trickof photosynthesis, is to convert the energy

of those 12 year old- i mean, electrons, into something that theplant can use. we are literally going to be spending theentire rest of the video talking about that. i hope that that's ok with you. this first chlorophyll is not on its own here,it's part of an insanely complicated complex of proteins, lipids, and other molecules calledphotosystem ii that contains at least 99 different chemicals including over 30 individual chlorophyllmolecules. this is the first of four protein complexesthat plants need for the light dependent reactions. and if you think it's complicated that wecall the first complex photosystem ii instead

of photosystem i, then you're welcome tocall it by its full name, plastoquinone oxidoreductase. oh, no? you don't want to call it that? right then, photosystem ii, or, if you wantto be brief, psii. psii and indeed all of the protein complexesin the light-dependent reactions, straddle the membrane of the thylakoids in the chloroplasts. that excited electron is now going to go ona journey designed to extract all of its new energy and convert that energy into usefulstuff. this is called the electron transport chain, in which energized electrons lose theirenergy in a series of reactions that capture the energy necessary to keep life living.

psii's chlorophyll now has this electronthat is so excited that, when a special protein designed specifically for stealing electronsshows up, the electron actually leaps off of the chlorophyllmolecule onto the protein, which we call a mobile electron carrier because it's... ...a mobile electron carrier. the chlorophyll then freaks out like a motherwho has just had her 12 year old daughter abducted by a teen idol and is like "whatdo i do to fix this problem!" and then it, in cooperation with the restof psii does something so amazing and important that i can barely believe that it keeps happeningevery 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 thatwe can breathe. brief interjection: next time someone saysthat they don't like it when there are chemicals in their food, please remind them that alllife is made of chemicals and would they please stoppretending that the word chemical is somehow a synonym for carcinogen!

because, i mean, think about how chlorophyllfeels when you say that! it spends all of it's time and energy creating the air webreathe and then we're like "ew! chemicals are so gross!" now, remember, all energized electrons frompsii have been picked up by electron carriers and are now being transported onto our secondprotein complex the cytochrome complex! this little guy does two things...one, itserves as an intermediary between psii and ps i and, two, uses a bit of theenergy from the electron to pump another proton into the thylakoid.

so the thylakoid's starting to fill up withprotons. we've created some by splitting water, and we moved one in using the cytochrome complex.but why are we doing this? well...basically, what we're doing, is chargingthe thylakoid like a battery. by pumping the thylakoid full of protons,we're creating a concentration gradient. the protons then naturally want to get theheck away from each other, and so they push their way through an enzyme straddling thethylakoid membrane called atp synthase, and that enzyme uses that energy to pack an inorganicphosphate onto adp, making atp: the big daddy of cellular energy. all this moving along the electron transportchain requires energy, and as you might expect

electrons are entering lower and lower energystates as we move along. this makes sense when you think about it. it's been a longwhile since those photons zapped us, and we've beenpumping hydrogen ions to create atp and splitting water and jumping onto different moleculesand i'm tired just talking about it. luckily, as 450 million years of evolutionwould have it, our electron is now about to be re-energized upon delivery to photosystem i! so, ps i is a similar mix of proteins andchlorophyll molecules that we saw in psii, but with some different products. after a couple of photons re-excite a coupleof electrons, the electrons pop off, and hitch

a ride onto another electron carrier. this time, all of that energy will be usedto help make nadph, which, like atp, exists solely to carry energy around.here, yet another enzyme helps combine two electrons and one hydrogenion with a little something called nadp+. as you may recall from our recent talk aboutrespiration, there are these sort of distant cousins of b vitamins that are crucialto energy conversion. and in photosynthesis, it's nadp+, and when ittakes on those 2 electrons and one hydrogen ion, it becomes nadph. so, what we're left with now, after thelight dependent reactions is chemical energy

in the form of atps and nadphs. and also ofcourse, we should not forget the most useful useless byproduct in the history ofuseless byproducts...oxygen. if anyone needs a potty break, now would bea good time...or if you want to go re-watch that rather long and complicatedbit about light dependent reactions, go ahead and do that...it'snot simple, and it's not going to get any simpler from here. because now we're moving alongto the calvin cycle! the calvin cycle is sometimes called the darkreactions, which is kind of a misnomer, because they generally don't occur in the dark. theyoccur in the day along with the rest of the

reactions, but they don't require energyfrom photons. so it's more proper to say light-independent. or, if you're feelingnon-descriptive...just say stage 2. stage 2 is all about using the energy fromthose atps and nadphs that we created in stage 1 to produce somethingactually useful for the plant. the calvin cycle begins in the stroma, theempty space in the chloroplast, if you remember correctly. and this phase is called carbonfixation 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 onlyis 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 rubp with the help of an enzymecalled ribulose 1,5 bisphosphate carboxylase oxidase, which we generallyshorten to rubisco. i'm in the chair again! excellent! this time for a biolo-graphy of rubisco. once upon a time, a one-celled organism waslike "man, i need more carbon so i can make more little me's so i can take over thewhole world." luckily for that little organism, there wasa 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'tparticularly good at its job, but it was a

heck of a lot better than just hoping to runinto some chemically formed organic carbon, so the organism just made a ton of it to makeup for how bad it was. not only did the little plant stick with it,it took over the entire planet, rapidly becoming the dominant form of life. slowly, through other reactions, known asthe 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 theatmosphere, started getting confused. as often as half the time rubisco startedslicing ribulose bisphosphate with oxygen instead of co2, creating a toxic byproductthat plants then had to deal with in creative

and specialized ways. this byproduct, called phosphogycolate, isbelieved 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-inon 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 40 billion tons of rubisco on the planet) and plants just deal with that toxic byproduct. another example, my friends, of unintelligentdesign.

back to the cycle! so ribulose bisphosphate gets a co2 slammedonto it and then immediately the whole thing gets crazy unstable. the only way to regainstability is for this new six-carbon chain to break apart creating two molecules of3-phosphoglycerate, and these arethe 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 slamsa phosphate group onto the 3-phosphoglycerate,

and then nadph pops 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 forstructure, starch for long-term storage. and because of this, g3p is considered theultimate product of photosynthesis. however, unfortunately, this is not the end.we need 5 g3ps to regenerate the 3 rubps that we started with. we also need 9 moleculesof atp and 6 molecules of nadph, so with all of these chemical reactions, all of this chemicalenergy, we can convert 3 rubps into 6 g3ps but onlyone of those g3ps gets to leave the cycle,

the other g3ps, of course, being needed toregenerate the original 3 ribulose bisphosphates. that regeneration is the last phase of thecalvin cycle. and that is how plants turn sunlight, water,and carbon dioxide into every living thing you've ever talked to, played with, climbedon, loved, hated, or eaten. not bad, plants. i hope you understand. if you don't, not onlydo we have some selected references below that you can check out, but of course, youcan go re-watch anything that you didn't get and hopefully, upon review, it will make alittle bit more sense. thank you for watching. if you have questions,please leave them down in the comments below.