WEBVTT 00:00:00.089 --> 00:00:05.800 Photosynthesis! It is not some kind of abstract scientific thing. You would be dead without 00:00:05.800 --> 00:00:11.269 plants and their magical- nay, SCIENTIFIC ability to convert sunlight, carbon dioxide 00:00:11.269 --> 00:00:15.480 and water into glucose and pure, delicious oxygen. 00:00:15.480 --> 00:00:20.619 This happens exclusively through photosynthesis, a process that was developed 450 million years 00:00:20.619 --> 00:00:23.550 ago and actually rather sucks. 00:00:23.550 --> 00:00:28.949 It's complicated, inefficient and confusing. But you are committed to having a better, 00:00:28.949 --> 00:00:33.050 deeper understanding of our world! Or, more probably, you'd like to do well on your 00:00:33.050 --> 00:00:39.399 test...so let's delve. 00:00:39.399 --> 00:00:46.399 There are two sorts of reactions in Photosynthesis...light dependent reactions, and light independent 00:00:48.910 --> 00:00:52.430 reactions, and you've probably already figured out the difference between those two, so that's 00:00:52.430 --> 00:00:57.850 nice. The light independent reactions are called the "calvin cycle" 00:00:57.850 --> 00:00:59.280 no...no...no...no...YES! THAT Calvin Cycle. 00:00:59.280 --> 00:01:00.800 00:01:00.800 --> 00:01:04.949 Photosynthesis is basically respiration in reverse, and we've already covered respiration, 00:01:04.949 --> 00:01:09.359 so maybe you should just go watch that video backwards. Or you can keep watching this one. 00:01:09.359 --> 00:01:09.880 Either way. 00:01:09.880 --> 00:01:14.109 I've already talked about what photosynthesis needs in order to work: water, carbon dioxide 00:01:14.109 --> 00:01:14.829 and sunlight. 00:01:14.829 --> 00:01:16.520 So, how do they get those things? 00:01:16.520 --> 00:01:19.869 First, water. Let's assume that we're talking about a vascular plant here, that's 00:01:19.869 --> 00:01:23.789 the kind of plant that has pipe-like tissues that conduct water, minerals and other materials 00:01:23.789 --> 00:01:25.130 to different parts of the plant. 00:01:25.130 --> 00:01:27.279 These are like trees and grasses and flowering plants. 00:01:27.279 --> 00:01:29.560 In this case the roots of the plants absorb water 00:01:29.560 --> 00:01:32.959 and bring it to the leaves through tissues called xylem. 00:01:32.959 --> 00:01:38.639 Carbon dioxide gets in and oxygen gets out through tiny pores in the leaves called stomata. 00:01:38.639 --> 00:01:42.479 It's actually surprisingly important that plants keep oxygen levels low inside of their 00:01:42.479 --> 00:01:44.919 leaves for reasons that we will get into later. 00:01:44.919 --> 00:01:51.139 And finally, individual photons from the Sun are absorbed in the plant by a pigment called 00:01:51.139 --> 00:01:51.450 chlorophyll. 00:01:51.450 --> 00:01:55.599 Alright, you remember plant cells? If not, you can go watch the video where we spend 00:01:55.599 --> 00:01:57.229 the whole time talking about plant cells. 00:01:57.229 --> 00:02:00.669 One thing that plant cells have that animal cells don't... plastids. 00:02:00.669 --> 00:02:02.299 And what is the most important plastid? 00:02:02.299 --> 00:02:07.469 The chloroplast! Which is not, as it is sometimes portrayed, just a big fat sac of chlorophyl. 00:02:07.469 --> 00:02:09.780 It's got complicated internal structure. 00:02:09.780 --> 00:02:14.170 Now, the chlorophyll is stashed in membranous sacs called thylakoids. The thykaloids are 00:02:14.170 --> 00:02:18.689 stacked into grana. Inside of the thykaloid is the lumen, and outside the 00:02:18.689 --> 00:02:22.040 thykaloid (but still inside the chloroplast) is the stroma. 00:02:22.040 --> 00:02:25.250 The thylakoid membranes are phospholipid bilayers, which, if you remember 00:02:25.250 --> 00:02:29.810 means they're really good at maintaining concentration gradients of ions, 00:02:29.810 --> 00:02:33.280 proteins and other things. This means keeping the concentration higher on one side 00:02:33.280 --> 00:02:37.879 than the other of the membrane. You're going to need to know all of these things, I'm sorry. 00:02:37.879 --> 00:02:40.780 Now that we've taken that little tour of the Chloroplast, it's time to get down to 00:02:40.780 --> 00:02:42.269 the actual chemistry. 00:02:42.269 --> 00:02:46.700 First thing that happens: A photon created by the fusion reactions of our sun is about 00:02:46.700 --> 00:02:50.040 to end its 93 million mile journey by slapping into a molecule of cholorophyll. 00:02:50.040 --> 00:02:54.730 This kicks off stage one, the light-dependent reactions proving 00:02:54.730 --> 00:02:58.420 that, yes, nearly all life on our planet is fusion-powered. 00:02:58.420 --> 00:03:03.709 When Chlorophyll gets hit by that photon, an electron absorbs that energy and gets excited. 00:03:03.709 --> 00:03:08.069 This is the technical term for electrons gaining energy and not having anywhere to put it and 00:03:08.069 --> 00:03:11.980 when it's done by a photon it's called photoexcitation, but let's just imagine, 00:03:11.980 --> 00:03:13.909 for the moment anyway, that every photon is whatever 00:03:13.909 --> 00:03:18.879 dreamy young man 12 year old girls are currently obsessed with, and electrons are 12 year old girls. 00:03:18.879 --> 00:03:23.019 The trick now, and the entire trick of photosynthesis, is to convert the energy 00:03:23.019 --> 00:03:23.909 of those 12 year old- 00:03:23.909 --> 00:03:27.620 I mean, electrons, into something that the plant can use. 00:03:27.620 --> 00:03:31.459 We are literally going to be spending the entire rest of the video talking about that. 00:03:31.459 --> 00:03:33.480 I hope that that's ok with you. 00:03:33.480 --> 00:03:39.030 This first Chlorophyll is not on its own here, it's part of an insanely complicated complex 00:03:39.030 --> 00:03:45.239 of proteins, lipids, and other molecules called Photosystem II that contains at least 99 different 00:03:45.239 --> 00:03:49.430 chemicals including over 30 individual chlorophyll molecules. 00:03:49.430 --> 00:03:54.359 This is the first of four protein complexes that plants need for the light dependent reactions. 00:03:54.359 --> 00:03:59.120 And if you think it's complicated that we call the first complex photosystem II instead 00:03:59.120 --> 00:04:04.629 of Photosystem I, then you're welcome to call it by its full name, plastoquinone oxidoreductase. 00:04:04.629 --> 00:04:07.810 Oh, no? You don't want to call it that? 00:04:07.810 --> 00:04:11.889 Right then, photosystem II, or, if you want to be brief, PSII. 00:04:11.889 --> 00:04:15.760 PSII and indeed all of the protein complexes in the light-dependent reactions, straddle 00:04:15.760 --> 00:04:19.228 the membrane of the thylakoids in the chloroplasts. 00:04:19.228 --> 00:04:23.569 That excited electron is now going to go on a journey designed to extract all of its new 00:04:23.569 --> 00:04:28.889 energy and convert that energy into useful stuff. This is called the electron transport 00:04:28.889 --> 00:04:33.080 chain, in which energized electrons lose their energy in a series of reactions that capture 00:04:33.080 --> 00:04:36.440 the energy necessary to keep life living. 00:04:36.440 --> 00:04:41.940 PSII's Chlorophyll now has this electron that is so excited that, when a special protein 00:04:41.940 --> 00:04:44.300 designed specifically for stealing electrons shows up, 00:04:44.300 --> 00:04:49.580 the electron actually leaps off of the chlorophyll molecule onto the protein, which we call a 00:04:49.580 --> 00:04:51.389 mobile electron carrier because it's... 00:04:51.389 --> 00:04:53.380 ...a mobile electron carrier. 00:04:53.380 --> 00:04:57.800 The Chlorophyll then freaks out like a mother who has just had her 12 year old daughter 00:04:57.800 --> 00:05:02.270 abducted by a teen idol and is like "WHAT DO I DO TO FIX THIS PROBLEM!" 00:05:02.270 --> 00:05:08.110 and then it, in cooperation with the rest of PSII does something so amazing and important 00:05:08.110 --> 00:05:11.520 that I can barely believe that it keeps happening every day. 00:05:11.520 --> 00:05:16.550 It splits that ultra-stable molecule, H2O, stealing one of its electrons, to replenish 00:05:16.550 --> 00:05:17.910 the one it lost. 00:05:17.910 --> 00:05:20.099 The byproducts of this water splitting? 00:05:20.099 --> 00:05:25.930 Hydrogen ions, which are just single protons, and oxygen. Sweet, sweet oxygen. 00:05:25.930 --> 00:05:29.580 This reaction, my friends, is the reason that we can breathe. 00:05:29.580 --> 00:05:33.610 Brief interjection: Next time someone says that they don't like it when there are chemicals 00:05:33.610 --> 00:05:37.300 in their food, please remind them that all life is 00:05:37.300 --> 00:05:41.210 made of chemicals and would they please stop pretending that the word chemical is somehow 00:05:41.210 --> 00:05:42.650 a synonym for carcinogen! 00:05:42.650 --> 00:05:48.740 Because, I mean, think about how chlorophyll feels when you say that! It spends all of 00:05:48.740 --> 00:05:52.080 it's time and energy creating the air we breathe and then we're like 00:05:52.080 --> 00:05:54.490 "EW! CHEMICALS ARE SO GROSS!" 00:05:54.490 --> 00:06:00.759 Now, remember, all energized electrons from PSII have been picked up by electron carriers 00:06:00.759 --> 00:06:03.919 and are now being transported onto our second protein complex 00:06:03.919 --> 00:06:05.199 the Cytochrome Complex! 00:06:05.199 --> 00:06:08.860 This little guy does two things...one, it serves as an intermediary between 00:06:08.860 --> 00:06:13.699 PSII and PS I and, two, uses a bit of the energy from the electron to 00:06:13.699 --> 00:06:16.419 pump another proton into the thylakoid. 00:06:16.419 --> 00:06:20.580 So the thylakoid's starting to fill up with protons. We've created some by splitting water, 00:06:20.580 --> 00:06:24.539 and we moved one in using the Cytochrome complex. But why are we doing this? 00:06:24.539 --> 00:06:27.780 Well...basically, what we're doing, is charging the Thylakoid like a battery. 00:06:27.780 --> 00:06:31.360 By pumping the thylakoid full of protons, we're creating a concentration gradient. 00:06:31.360 --> 00:06:34.280 The protons then naturally want to get the heck away from each other, and so they push 00:06:34.280 --> 00:06:39.319 their way through an enzyme straddling the thylakoid membrane called ATP Synthase, and 00:06:39.319 --> 00:06:46.169 that enzyme uses that energy to pack an inorganic phosphate onto ADP, making ATP: the big daddy 00:06:46.169 --> 00:06:47.000 of cellular energy. 00:06:47.000 --> 00:06:51.389 All this moving along the electron transport chain requires energy, and as you might expect 00:06:51.389 --> 00:06:55.610 electrons are entering lower and lower energy states as we move along. This makes sense 00:06:55.610 --> 00:06:57.439 when you think about it. It's been a long while since 00:06:57.439 --> 00:07:02.030 those photons zapped us, and we've been pumping hydrogen ions to create ATP and splitting 00:07:02.030 --> 00:07:06.330 water and jumping onto different molecules and I'm tired just talking about it. 00:07:06.330 --> 00:07:11.509 Luckily, as 450 million years of evolution would have it, our electron is now about to 00:07:11.509 --> 00:07:14.259 be re-energized upon delivery to Photosystem I! 00:07:14.259 --> 00:07:18.879 So, PS I is a similar mix of proteins and chlorophyll molecules that we saw in PSII, 00:07:18.879 --> 00:07:20.699 but with some different products. 00:07:20.699 --> 00:07:25.030 After a couple of photons re-excite a couple of electrons, the electrons pop off, and hitch 00:07:25.030 --> 00:07:27.289 a ride onto another electron carrier. 00:07:27.289 --> 00:07:33.199 This time, all of that energy will be used to help make NADPH, which, like ATP, exists 00:07:33.199 --> 00:07:37.060 solely to carry energy around. Here, yet another enzyme 00:07:37.060 --> 00:07:41.460 helps combine two electrons and one hydrogen ion with a little something called NADP+. 00:07:41.460 --> 00:07:44.819 As you may recall from our recent talk about respiration, there are these sort of 00:07:44.819 --> 00:07:49.500 distant cousins of B vitamins that are crucial to energy conversion. And in photosynthesis, 00:07:49.500 --> 00:07:52.479 it's NADP+, and when it takes on those 2 electrons 00:07:52.479 --> 00:07:55.139 and one hydrogen ion, it becomes NADPH. 00:07:55.139 --> 00:07:59.879 So, what we're left with now, after the light dependent reactions is chemical energy 00:07:59.879 --> 00:08:04.639 in the form of ATPs and NADPHs. And also of course, we should not forget the most useful 00:08:04.639 --> 00:08:08.009 useless byproduct in the history of useless byproducts...oxygen. 00:08:08.009 --> 00:08:12.419 If anyone needs a potty break, now would be a good time...or if you want to go re-watch 00:08:12.419 --> 00:08:15.009 that rather long and complicated bit about light 00:08:15.009 --> 00:08:19.810 dependent reactions, go ahead and do that...it's not simple, and it's not going to get any 00:08:19.810 --> 00:08:20.849 simpler from here. 00:08:20.849 --> 00:08:24.120 Because now we're moving along to the Calvin Cycle! 00:08:24.120 --> 00:08:28.539 The Calvin Cycle is sometimes called the dark reactions, which is kind of a misnomer, because 00:08:28.539 --> 00:08:33.039 they generally don't occur in the dark. They occur in the day along with the rest of the 00:08:33.039 --> 00:08:37.820 reactions, but they don't require energy from photons. So it's more proper to say 00:08:37.820 --> 00:08:43.929 light-independent. Or, if you're feeling non-descriptive...just say Stage 2. 00:08:43.929 --> 00:08:47.920 Stage 2 is all about using the energy from those ATPs and NADPHs that we created in 00:08:47.920 --> 00:08:50.480 Stage 1 to produce something actually useful for the plant. 00:08:50.480 --> 00:08:55.460 The Calvin Cycle begins in the stroma, the empty space in the chloroplast, if you remember 00:08:55.460 --> 00:09:00.720 correctly. And this phase is called carbon fixation because...yeah, we're about to 00:09:00.720 --> 00:09:06.100 fix a CO2 molecule onto our starting point, Ribulose Bisphosphate or RuBP, which is always 00:09:06.100 --> 00:09:09.950 around in the chloroplast because, not only is it the starting point of the Calvin Cycle, 00:09:09.950 --> 00:09:13.180 it's also the end-point... which is why it's a cycle. 00:09:13.180 --> 00:09:19.310 CO2 is fixed to RuBP with the help of an enzyme called ribulose 1,5 bisphosphate carboxylase 00:09:19.310 --> 00:09:26.310 oxidase, which we generally shorten to RuBisCo. 00:09:28.400 --> 00:09:30.900 I'm in the chair again! Excellent! 00:09:30.900 --> 00:09:34.400 This time for a Biolo-graphy of RuBisCo. 00:09:34.400 --> 00:09:39.380 Once upon a time, a one-celled organism was like "Man, I need more carbon so I can make 00:09:39.380 --> 00:09:42.700 more little me's so I can take over the whole world." 00:09:42.700 --> 00:09:47.400 Luckily for that little organism, there was a lot of CO2 in the atmosphere, and so it 00:09:47.400 --> 00:09:53.630 evolved an enzyme that could suck up that CO2 and convert inorganic carbon into organic carbon. 00:09:53.630 --> 00:09:53.880 00:09:53.820 --> 00:09:57.880 This enzyme was called RuBisCo, and it wasn't particularly good at its job, but it was a 00:09:57.880 --> 00:10:01.900 heck of a lot better than just hoping to run into some chemically formed organic carbon, 00:10:01.900 --> 00:10:05.810 so the organism just made a ton of it to make up for how bad it was. 00:10:05.810 --> 00:10:10.430 Not only did the little plant stick with it, it took over the entire planet, rapidly becoming 00:10:10.430 --> 00:10:12.050 the dominant form of life. 00:10:12.050 --> 00:10:16.210 Slowly, through other reactions, known as the light dependent reactions, plants increased 00:10:16.210 --> 00:10:21.260 the amount of oxygen in the atmosphere. RuBisCo, having been designed in a world with 00:10:21.260 --> 00:10:24.870 tiny amounts of oxygen in the atmosphere, started getting confused. 00:10:24.870 --> 00:10:29.800 As often as half the time RuBisCo started slicing Ribulose Bisphosphate with Oxygen 00:10:29.800 --> 00:10:34.450 instead of CO2, creating a toxic byproduct that plants then had to deal with in creative 00:10:34.450 --> 00:10:35.670 and specialized ways. 00:10:35.670 --> 00:10:40.000 This byproduct, called phosphogycolate, is believed to tinker with some enzyme functions, 00:10:40.000 --> 00:10:44.450 including some involved in the Calvin cycle, so plants have to make other enzymes that 00:10:44.450 --> 00:10:49.280 break it down into an amino acid (glycine), and some compounds that are actually useful 00:10:49.280 --> 00:10:50.240 to the Calvin cycle. 00:10:50.240 --> 00:10:54.670 But plants had already sort of gone all-in on the RuBisCo strategy and, to this day, 00:10:54.670 --> 00:10:59.150 they have to produce huge amounts of it (scientists estimate that at any given time there 00:10:59.150 --> 00:11:05.970 are about 40 billion tons of RuBisCo on the planet) and plants just deal with that toxic byproduct. 00:11:05.970 --> 00:11:10.090 Another example, my friends, of unintelligent design. 00:11:10.090 --> 00:11:11.100 Back to the cycle! 00:11:11.100 --> 00:11:15.110 So Ribulose Bisphosphate gets a CO2 slammed onto it and then immediately the whole thing 00:11:15.110 --> 00:11:19.670 gets crazy unstable. The only way to regain stability is for this new six-carbon chain 00:11:19.670 --> 00:11:26.020 to break apart creating two molecules of 3-Phosphoglycerate, and these are the first stable products of the calvin cycle. 00:11:26.020 --> 00:11:26.820 00:11:26.820 --> 00:11:29.780 For reasons that will become clear in a moment, we're actually going to do this to three 00:11:29.780 --> 00:11:31.440 molecules of RuBP. 00:11:31.440 --> 00:11:33.170 Now we enter the second phase, 00:11:33.170 --> 00:11:33.610 Reduction. 00:11:33.610 --> 00:11:38.710 Here, we need some energy. So some ATP slams a phosphate group onto the 3-Phosphoglycerate, 00:11:38.710 --> 00:11:45.050 and then NADPH pops some electrons on and, voila, we have two molecules of Glyceraldehyde 00:11:45.050 --> 00:11:50.200 3-Phosphate, or G3P, this is a high-energy, 3-carbon compound that plants can convert 00:11:50.200 --> 00:11:53.700 into pretty much any carbohydrate. Like glucose 00:11:53.700 --> 00:11:58.190 for short term energy storage, cellulose for structure, starch for long-term storage. 00:11:58.190 --> 00:12:02.960 And because of this, G3P is considered the ultimate product of photosynthesis. 00:12:02.960 --> 00:12:08.810 However, unfortunately, this is not the end. We need 5 G3Ps to regenerate the 3 RuBPs that 00:12:08.810 --> 00:12:15.690 we started with. We also need 9 molecules of ATP and 6 molecules of NADPH, so with all 00:12:15.690 --> 00:12:17.710 of these chemical reactions, all of this chemical energy, 00:12:17.710 --> 00:12:24.110 we can convert 3 RuBPs into 6 G3Ps but only one of those G3Ps gets to leave the cycle, 00:12:24.110 --> 00:12:29.550 the other G3Ps, of course, being needed to regenerate the original 3 Ribulose Bisphosphates. 00:12:29.550 --> 00:12:32.860 That regeneration is the last phase of the Calvin Cycle. 00:12:32.860 --> 00:12:38.040 And that is how plants turn sunlight, water, and carbon dioxide into every living thing 00:12:38.040 --> 00:12:44.480 you've ever talked to, played with, climbed on, loved, hated, or eaten. Not bad, plants. 00:12:44.480 --> 00:12:49.390 I hope you understand. If you don't, not only do we have some selected references below 00:12:49.390 --> 00:12:54.110 that you can check out, but of course, you can go re-watch anything that you didn't get 00:12:54.110 --> 00:12:57.510 and hopefully, upon review, it will make a little bit more sense. 00:12:57.510 --> 00:12:59.970 Thank you for watching. If you have questions, please leave them down in the comments below.