Photosynthesis

- 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 test. So let's delve. (upbeat energetic music) 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 the 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. Alright, you remember plant cells? If not, you can go watch the video where we spent 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 plastid? The chloroplast. Which is not, as it is sometimes portrayed, just a big fat sac of chlorophyll. It's got complicated internal structure. Now, the chlorophyll is stashed in 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 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 the 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 a 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 its 93 million mile journey by slapping into a molecule of chlorophyll. This kicks off stage one, the light dependent reactions. Proving that, yes, nearly all life on our planet is fusion-powered. When chlorophyll gets hit by that photon, an 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 photoexcitation. But let's just imagine for the moment, anyway, that every photon is whatever dreamy young man 12 year old girls are currently obsessed with. And electrons are 12 year old girls. The trick now, and the entire trick of photosynthesis, is to convert the energy of those 12 year olds, 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 two 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 two instead of photosystem one, then you're welcome to call it by its full name which is plastoquinone oxidoreductase. Oh no, you don't want to call it that? Right then, photosystem two. Or if you want to be brief, PS two. PS two, and indeed, all of the protein complexes in the light dependent reactions straddle the membrane of the thylakoids in the chloroplast. Now, that exited 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 a series of reactions that capture the energy necessary to keep life living. So PS two'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. The chlorophyll then freaks out like a mother who has just had her 12 year old daughter abducted by a teen idol, as is like what do I do to fix this problem? And then it, in cooperation with the rest of photosystem two, does something so amazing and important that I can barely believe that it keeps happening everyday, 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 breath. 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. Beause, I mean, think about how chlorophyll feels when you say that. It spends all of its time and energy creating the air we breath and we're like ew, chemicals are so gross. Now, remember all energized electrons from PS two have been picked up by electron carriers and now are being transported to the second protein complex, the Cytochrome Complex. This little guy does two things: one, it serves as an intermediary between PS two and PS one. And two, it uses a little bit of that energy from the electron to pump another proton into the thylakoid. So the thylakoid's starting to fill up with protons. We've created some by splitting water, and we moved one using the Cytochrome Complex, but why are we doing this? Well, basically what we're doing is charging the thlakoid like a battery. By pumping the thylakoid with protons, we're creating a concentration gradient. The protons then naturally wanna 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 cellular energy. All of this moving along the electron transport train 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. We've been pumping hydrogen ions to create ATP, it's splitting water and jumping onto different molecules, and I'm 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 one. So PS one is a similar mix of proteins and chlorophyll molecules that we saw in PS two but with some different products. After a couple of photons re-excite a couple of the electrons, the electrons pop off and hitch a ride onto another electron carrier. This time all of the 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 plus. As you may recall from our recent talk about respiration, they're sort of the distant cousins of B vitamins that are crucial to energy conversion. In photosynthesis, it's NADP plus 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 ATPs and NADPHs. 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 wanna go re-watch that rather long and complicated bit about light dependent reactions, go ahead and do that. It's not simple and it's not gonna 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 miss misnomer because they generally don't occur in the dark. They 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 ATPs and NADPHs 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 CO two 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 RuBP with the help of an enzyme called ribulose one five bisphosphate carboxylase oxidase which we generally shorten to RuBisCO. (upbeat piano music) I'm in the chair again, excellent. This time for a biolo-graphy 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 plant 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 byproduct that plants had to deal with in creative and specialized ways. This byproduct called phosphogycolate is 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 amino acid, glycine, and some compounds that are actually useful to the Calvin Cycle. But, plants that are already sort of gone all in on the RuBisCO strategy, 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 unintelligent design. Back to the cycle. So ribulose bisphosphate gets a 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 three phosphoglycerate. And these are the first stable products of the Calvin Cycle. For reasons that will become clear in a moment, we're actually gonna do this to three molecules of RuBP. Now, we enter the second phase, reduction. Here we need some energy. So some ATPs slams the phosphate group onto the three phosphoglycerate and then NADPH pops some electrons on and wallah, we have two molecules of glyceraldehyde three phosphate or G three P. This is a high energy three carbon compound that plants can convert into pretty much any carbohydrate like glucose for short term energy storage, cellulose for structures, starch for long term storage. And because of this, G three P is considered the ultimate product of photosynthesis. However, unfortunately this is not the end, we need five G three Ps to regenerate the three RuBPs 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 RuBPs into six G three Ps but only one of those G three Ps gets to leave the cycle. The other G three Ps, of course being needed to regenerate the original three ribulose bisphosphates. 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.