1 00:00:00,089 --> 00:00:05,800 Photosynthesis! It is not some kind of abstract scientific thing. You would be dead without 2 00:00:05,800 --> 00:00:11,269 plants and their magical- nay, SCIENTIFIC ability to convert sunlight, carbon dioxide 3 00:00:11,269 --> 00:00:15,480 and water into glucose and pure, delicious oxygen. 4 00:00:15,480 --> 00:00:20,619 This happens exclusively through photosynthesis, a process that was developed 450 million years 5 00:00:20,619 --> 00:00:23,550 ago and actually rather sucks. 6 00:00:23,550 --> 00:00:28,949 It's complicated, inefficient and confusing. But you are committed to having a better, 7 00:00:28,949 --> 00:00:33,050 deeper understanding of our world! Or, more probably, you'd like to do well on your 8 00:00:33,050 --> 00:00:39,399 test...so let's delve. 9 00:00:39,399 --> 00:00:46,399 There are two sorts of reactions in Photosynthesis...light dependent reactions, and light independent 10 00:00:48,910 --> 00:00:52,430 reactions, and you've probably already figured out the difference between those two, so that's 11 00:00:52,430 --> 00:00:57,850 nice. The light independent reactions are called the "calvin cycle" 12 00:00:57,850 --> 00:00:59,280 no...no...no...no...YES! THAT Calvin Cycle. 13 00:00:59,280 --> 00:01:00,800 14 00:01:00,800 --> 00:01:04,949 Photosynthesis is basically respiration in reverse, and we've already covered respiration, 15 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. 16 00:01:09,359 --> 00:01:09,880 Either way. 17 00:01:09,880 --> 00:01:14,109 I've already talked about what photosynthesis needs in order to work: water, carbon dioxide 18 00:01:14,109 --> 00:01:14,829 and sunlight. 19 00:01:14,829 --> 00:01:16,520 So, how do they get those things? 20 00:01:16,520 --> 00:01:19,869 First, water. Let's assume that we're talking about a vascular plant here, that's 21 00:01:19,869 --> 00:01:23,789 the kind of plant that has pipe-like tissues that conduct water, minerals and other materials 22 00:01:23,789 --> 00:01:25,130 to different parts of the plant. 23 00:01:25,130 --> 00:01:27,279 These are like trees and grasses and flowering plants. 24 00:01:27,279 --> 00:01:29,560 In this case the roots of the plants absorb water 25 00:01:29,560 --> 00:01:32,959 and bring it to the leaves through tissues called xylem. 26 00:01:32,959 --> 00:01:38,639 Carbon dioxide gets in and oxygen gets out through tiny pores in the leaves called stomata. 27 00:01:38,639 --> 00:01:42,479 It's actually surprisingly important that plants keep oxygen levels low inside of their 28 00:01:42,479 --> 00:01:44,919 leaves for reasons that we will get into later. 29 00:01:44,919 --> 00:01:51,139 And finally, individual photons from the Sun are absorbed in the plant by a pigment called 30 00:01:51,139 --> 00:01:51,450 chlorophyll. 31 00:01:51,450 --> 00:01:55,599 Alright, you remember plant cells? If not, you can go watch the video where we spend 32 00:01:55,599 --> 00:01:57,229 the whole time talking about plant cells. 33 00:01:57,229 --> 00:02:00,669 One thing that plant cells have that animal cells don't... plastids. 34 00:02:00,669 --> 00:02:02,299 And what is the most important plastid? 35 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. 36 00:02:07,469 --> 00:02:09,780 It's got complicated internal structure. 37 00:02:09,780 --> 00:02:14,170 Now, the chlorophyll is stashed in membranous sacs called thylakoids. The thykaloids are 38 00:02:14,170 --> 00:02:18,689 stacked into grana. Inside of the thykaloid is the lumen, and outside the 39 00:02:18,689 --> 00:02:22,040 thykaloid (but still inside the chloroplast) is the stroma. 40 00:02:22,040 --> 00:02:25,250 The thylakoid membranes are phospholipid bilayers, which, if you remember 41 00:02:25,250 --> 00:02:29,810 means they're really good at maintaining concentration gradients of ions, 42 00:02:29,810 --> 00:02:33,280 proteins and other things. This means keeping the concentration higher on one side 43 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. 44 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 45 00:02:40,780 --> 00:02:42,269 the actual chemistry. 46 00:02:42,269 --> 00:02:46,700 First thing that happens: A photon created by the fusion reactions of our sun is about 47 00:02:46,700 --> 00:02:50,040 to end its 93 million mile journey by slapping into a molecule of cholorophyll. 48 00:02:50,040 --> 00:02:54,730 This kicks off stage one, the light-dependent reactions proving 49 00:02:54,730 --> 00:02:58,420 that, yes, nearly all life on our planet is fusion-powered. 50 00:02:58,420 --> 00:03:03,709 When Chlorophyll gets hit by that photon, an electron absorbs that energy and gets excited. 51 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 52 00:03:08,069 --> 00:03:11,980 when it's done by a photon it's called photoexcitation, but let's just imagine, 53 00:03:11,980 --> 00:03:13,909 for the moment anyway, that every photon is whatever 54 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. 55 00:03:18,879 --> 00:03:23,019 The trick now, and the entire trick of photosynthesis, is to convert the energy 56 00:03:23,019 --> 00:03:23,909 of those 12 year old- 57 00:03:23,909 --> 00:03:27,620 I mean, electrons, into something that the plant can use. 58 00:03:27,620 --> 00:03:31,459 We are literally going to be spending the entire rest of the video talking about that. 59 00:03:31,459 --> 00:03:33,480 I hope that that's ok with you. 60 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 61 00:03:39,030 --> 00:03:45,239 of proteins, lipids, and other molecules called Photosystem II that contains at least 99 different 62 00:03:45,239 --> 00:03:49,430 chemicals including over 30 individual chlorophyll molecules. 63 00:03:49,430 --> 00:03:54,359 This is the first of four protein complexes that plants need for the light dependent reactions. 64 00:03:54,359 --> 00:03:59,120 And if you think it's complicated that we call the first complex photosystem II instead 65 00:03:59,120 --> 00:04:04,629 of Photosystem I, then you're welcome to call it by its full name, plastoquinone oxidoreductase. 66 00:04:04,629 --> 00:04:07,810 Oh, no? You don't want to call it that? 67 00:04:07,810 --> 00:04:11,889 Right then, photosystem II, or, if you want to be brief, PSII. 68 00:04:11,889 --> 00:04:15,760 PSII and indeed all of the protein complexes in the light-dependent reactions, straddle 69 00:04:15,760 --> 00:04:19,228 the membrane of the thylakoids in the chloroplasts. 70 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 71 00:04:23,569 --> 00:04:28,889 energy and convert that energy into useful stuff. This is called the electron transport 72 00:04:28,889 --> 00:04:33,080 chain, in which energized electrons lose their energy in a series of reactions that capture 73 00:04:33,080 --> 00:04:36,440 the energy necessary to keep life living. 74 00:04:36,440 --> 00:04:41,940 PSII's Chlorophyll now has this electron that is so excited that, when a special protein 75 00:04:41,940 --> 00:04:44,300 designed specifically for stealing electrons shows up, 76 00:04:44,300 --> 00:04:49,580 the electron actually leaps off of the chlorophyll molecule onto the protein, which we call a 77 00:04:49,580 --> 00:04:51,389 mobile electron carrier because it's... 78 00:04:51,389 --> 00:04:53,380 ...a mobile electron carrier. 79 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 80 00:04:57,800 --> 00:05:02,270 abducted by a teen idol and is like "WHAT DO I DO TO FIX THIS PROBLEM!" 81 00:05:02,270 --> 00:05:08,110 and then it, in cooperation with the rest of PSII does something so amazing and important 82 00:05:08,110 --> 00:05:11,520 that I can barely believe that it keeps happening every day. 83 00:05:11,520 --> 00:05:16,550 It splits that ultra-stable molecule, H2O, stealing one of its electrons, to replenish 84 00:05:16,550 --> 00:05:17,910 the one it lost. 85 00:05:17,910 --> 00:05:20,099 The byproducts of this water splitting? 86 00:05:20,099 --> 00:05:25,930 Hydrogen ions, which are just single protons, and oxygen. Sweet, sweet oxygen. 87 00:05:25,930 --> 00:05:29,580 This reaction, my friends, is the reason that we can breathe. 88 00:05:29,580 --> 00:05:33,610 Brief interjection: Next time someone says that they don't like it when there are chemicals 89 00:05:33,610 --> 00:05:37,300 in their food, please remind them that all life is 90 00:05:37,300 --> 00:05:41,210 made of chemicals and would they please stop pretending that the word chemical is somehow 91 00:05:41,210 --> 00:05:42,650 a synonym for carcinogen! 92 00:05:42,650 --> 00:05:48,740 Because, I mean, think about how chlorophyll feels when you say that! It spends all of 93 00:05:48,740 --> 00:05:52,080 it's time and energy creating the air we breathe and then we're like 94 00:05:52,080 --> 00:05:54,490 "EW! CHEMICALS ARE SO GROSS!" 95 00:05:54,490 --> 00:06:00,759 Now, remember, all energized electrons from PSII have been picked up by electron carriers 96 00:06:00,759 --> 00:06:03,919 and are now being transported onto our second protein complex 97 00:06:03,919 --> 00:06:05,199 the Cytochrome Complex! 98 00:06:05,199 --> 00:06:08,860 This little guy does two things...one, it serves as an intermediary between 99 00:06:08,860 --> 00:06:13,699 PSII and PS I and, two, uses a bit of the energy from the electron to 100 00:06:13,699 --> 00:06:16,419 pump another proton into the thylakoid. 101 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, 102 00:06:20,580 --> 00:06:24,539 and we moved one in using the Cytochrome complex. But why are we doing this? 103 00:06:24,539 --> 00:06:27,780 Well...basically, what we're doing, is charging the Thylakoid like a battery. 104 00:06:27,780 --> 00:06:31,360 By pumping the thylakoid full of protons, we're creating a concentration gradient. 105 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 106 00:06:34,280 --> 00:06:39,319 their way through an enzyme straddling the thylakoid membrane called ATP Synthase, and 107 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 108 00:06:46,169 --> 00:06:47,000 of cellular energy. 109 00:06:47,000 --> 00:06:51,389 All this moving along the electron transport chain requires energy, and as you might expect 110 00:06:51,389 --> 00:06:55,610 electrons are entering lower and lower energy states as we move along. This makes sense 111 00:06:55,610 --> 00:06:57,439 when you think about it. It's been a long while since 112 00:06:57,439 --> 00:07:02,030 those photons zapped us, and we've been pumping hydrogen ions to create ATP and splitting 113 00:07:02,030 --> 00:07:06,330 water and jumping onto different molecules and I'm tired just talking about it. 114 00:07:06,330 --> 00:07:11,509 Luckily, as 450 million years of evolution would have it, our electron is now about to 115 00:07:11,509 --> 00:07:14,259 be re-energized upon delivery to Photosystem I! 116 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, 117 00:07:18,879 --> 00:07:20,699 but with some different products. 118 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 119 00:07:25,030 --> 00:07:27,289 a ride onto another electron carrier. 120 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 121 00:07:33,199 --> 00:07:37,060 solely to carry energy around. Here, yet another enzyme 122 00:07:37,060 --> 00:07:41,460 helps combine two electrons and one hydrogen ion with a little something called NADP+. 123 00:07:41,460 --> 00:07:44,819 As you may recall from our recent talk about respiration, there are these sort of 124 00:07:44,819 --> 00:07:49,500 distant cousins of B vitamins that are crucial to energy conversion. And in photosynthesis, 125 00:07:49,500 --> 00:07:52,479 it's NADP+, and when it takes on those 2 electrons 126 00:07:52,479 --> 00:07:55,139 and one hydrogen ion, it becomes NADPH. 127 00:07:55,139 --> 00:07:59,879 So, what we're left with now, after the light dependent reactions is chemical energy 128 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 129 00:08:04,639 --> 00:08:08,009 useless byproduct in the history of useless byproducts...oxygen. 130 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 131 00:08:12,419 --> 00:08:15,009 that rather long and complicated bit about light 132 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 133 00:08:19,810 --> 00:08:20,849 simpler from here. 134 00:08:20,849 --> 00:08:24,120 Because now we're moving along to the Calvin Cycle! 135 00:08:24,120 --> 00:08:28,539 The Calvin Cycle is sometimes called the dark reactions, which is kind of a misnomer, because 136 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 137 00:08:33,039 --> 00:08:37,820 reactions, but they don't require energy from photons. So it's more proper to say 138 00:08:37,820 --> 00:08:43,929 light-independent. Or, if you're feeling non-descriptive...just say Stage 2. 139 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 140 00:08:47,920 --> 00:08:50,480 Stage 1 to produce something actually useful for the plant. 141 00:08:50,480 --> 00:08:55,460 The Calvin Cycle begins in the stroma, the empty space in the chloroplast, if you remember 142 00:08:55,460 --> 00:09:00,720 correctly. And this phase is called carbon fixation because...yeah, we're about to 143 00:09:00,720 --> 00:09:06,100 fix a CO2 molecule onto our starting point, Ribulose Bisphosphate or RuBP, which is always 144 00:09:06,100 --> 00:09:09,950 around in the chloroplast because, not only is it the starting point of the Calvin Cycle, 145 00:09:09,950 --> 00:09:13,180 it's also the end-point... which is why it's a cycle. 146 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 147 00:09:19,310 --> 00:09:26,310 oxidase, which we generally shorten to RuBisCo. 148 00:09:28,400 --> 00:09:30,900 I'm in the chair again! Excellent! 149 00:09:30,900 --> 00:09:34,400 This time for a Biolo-graphy of RuBisCo. 150 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 151 00:09:39,380 --> 00:09:42,700 more little me's so I can take over the whole world." 152 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 153 00:09:47,400 --> 00:09:53,630 evolved an enzyme that could suck up that CO2 and convert inorganic carbon into organic carbon. 154 00:09:53,630 --> 00:09:53,880 155 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 156 00:09:57,880 --> 00:10:01,900 heck of a lot better than just hoping to run into some chemically formed organic carbon, 157 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. 158 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 159 00:10:10,430 --> 00:10:12,050 the dominant form of life. 160 00:10:12,050 --> 00:10:16,210 Slowly, through other reactions, known as the light dependent reactions, plants increased 161 00:10:16,210 --> 00:10:21,260 the amount of oxygen in the atmosphere. RuBisCo, having been designed in a world with 162 00:10:21,260 --> 00:10:24,870 tiny amounts of oxygen in the atmosphere, started getting confused. 163 00:10:24,870 --> 00:10:29,800 As often as half the time RuBisCo started slicing Ribulose Bisphosphate with Oxygen 164 00:10:29,800 --> 00:10:34,450 instead of CO2, creating a toxic byproduct that plants then had to deal with in creative 165 00:10:34,450 --> 00:10:35,670 and specialized ways. 166 00:10:35,670 --> 00:10:40,000 This byproduct, called phosphogycolate, is believed to tinker with some enzyme functions, 167 00:10:40,000 --> 00:10:44,450 including some involved in the Calvin cycle, so plants have to make other enzymes that 168 00:10:44,450 --> 00:10:49,280 break it down into an amino acid (glycine), and some compounds that are actually useful 169 00:10:49,280 --> 00:10:50,240 to the Calvin cycle. 170 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, 171 00:10:54,670 --> 00:10:59,150 they have to produce huge amounts of it (scientists estimate that at any given time there 172 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. 173 00:11:05,970 --> 00:11:10,090 Another example, my friends, of unintelligent design. 174 00:11:10,090 --> 00:11:11,100 Back to the cycle! 175 00:11:11,100 --> 00:11:15,110 So Ribulose Bisphosphate gets a CO2 slammed onto it and then immediately the whole thing 176 00:11:15,110 --> 00:11:19,670 gets crazy unstable. The only way to regain stability is for this new six-carbon chain 177 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. 178 00:11:26,020 --> 00:11:26,820 179 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 180 00:11:29,780 --> 00:11:31,440 molecules of RuBP. 181 00:11:31,440 --> 00:11:33,170 Now we enter the second phase, 182 00:11:33,170 --> 00:11:33,610 Reduction. 183 00:11:33,610 --> 00:11:38,710 Here, we need some energy. So some ATP slams a phosphate group onto the 3-Phosphoglycerate, 184 00:11:38,710 --> 00:11:45,050 and then NADPH pops some electrons on and, voila, we have two molecules of Glyceraldehyde 185 00:11:45,050 --> 00:11:50,200 3-Phosphate, or G3P, this is a high-energy, 3-carbon compound that plants can convert 186 00:11:50,200 --> 00:11:53,700 into pretty much any carbohydrate. Like glucose 187 00:11:53,700 --> 00:11:58,190 for short term energy storage, cellulose for structure, starch for long-term storage. 188 00:11:58,190 --> 00:12:02,960 And because of this, G3P is considered the ultimate product of photosynthesis. 189 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 190 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 191 00:12:15,690 --> 00:12:17,710 of these chemical reactions, all of this chemical energy, 192 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, 193 00:12:24,110 --> 00:12:29,550 the other G3Ps, of course, being needed to regenerate the original 3 Ribulose Bisphosphates. 194 00:12:29,550 --> 00:12:32,860 That regeneration is the last phase of the Calvin Cycle. 195 00:12:32,860 --> 00:12:38,040 And that is how plants turn sunlight, water, and carbon dioxide into every living thing 196 00:12:38,040 --> 00:12:44,480 you've ever talked to, played with, climbed on, loved, hated, or eaten. Not bad, plants. 197 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 198 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 199 00:12:54,110 --> 00:12:57,510 and hopefully, upon review, it will make a little bit more sense. 200 00:12:57,510 --> 00:12:59,970 Thank you for watching. If you have questions, please leave them down in the comments below.