1 00:00:00,000 --> 00:00:01,988 [SQUEAKING] 2 00:00:01,988 --> 00:00:03,976 [RUSTLING] 3 00:00:03,976 --> 00:00:05,467 [CLICKING] 4 00:00:10,777 --> 00:00:11,360 PROFESSOR: OK. 5 00:00:11,360 --> 00:00:14,270 So today we're going to continue our discussion 6 00:00:14,270 --> 00:00:15,830 about photosynthesis. 7 00:00:15,830 --> 00:00:20,570 And last time, I introduced the reactions of photosynthesis 8 00:00:20,570 --> 00:00:24,110 and pointed out that really what we're talking about 9 00:00:24,110 --> 00:00:27,120 is oxidation reduction reactions. 10 00:00:27,120 --> 00:00:30,500 And so the net reaction of photosynthesis-- 11 00:00:30,500 --> 00:00:33,920 water plus carbon dioxide goes to carbohydrate and oxygen-- 12 00:00:33,920 --> 00:00:37,650 is really the opposite of burning wood. 13 00:00:37,650 --> 00:00:41,870 So if burning wood, carbon oxidation, is favorable, 14 00:00:41,870 --> 00:00:44,570 reversing this, obviously, needs energy input. 15 00:00:44,570 --> 00:00:50,090 And that energy input is light that's coming from the sun. 16 00:00:50,090 --> 00:00:54,620 And we started to discuss a bit about how this reaction works. 17 00:00:54,620 --> 00:00:56,840 And I mentioned that photosynthesis 18 00:00:56,840 --> 00:01:00,990 is broken down into light reactions and dark reactions. 19 00:01:00,990 --> 00:01:04,790 And so the light reactions are really 20 00:01:04,790 --> 00:01:08,540 along the same lines of the oxidative phosphorylation 21 00:01:08,540 --> 00:01:11,390 electron transport chain that we described. 22 00:01:11,390 --> 00:01:13,310 And that is basically using energy 23 00:01:13,310 --> 00:01:16,310 from the sun to use water as an electron donor 24 00:01:16,310 --> 00:01:20,990 to generate NADPH, as well as oxygen. 25 00:01:20,990 --> 00:01:27,020 In doing so, light from the sun makes this process favorable. 26 00:01:27,020 --> 00:01:29,270 That favorable electron transport can then 27 00:01:29,270 --> 00:01:35,060 be used to generate delta psi, delta pH, which can create ATP, 28 00:01:35,060 --> 00:01:37,910 do other work just like oxidative phosphorylation. 29 00:01:37,910 --> 00:01:40,280 Of course, photosynthesis comes first, oxidative 30 00:01:40,280 --> 00:01:42,780 phosphorylation came second, even though we taught it 31 00:01:42,780 --> 00:01:45,230 in the reverse order. 32 00:01:45,230 --> 00:01:48,630 And these reactions also produce NADPH, 33 00:01:48,630 --> 00:01:52,160 which can be used in the so-called dark reactions 34 00:01:52,160 --> 00:01:54,240 to synthesize carbohydrates. 35 00:01:54,240 --> 00:01:57,120 Now, the dark reactions, again, I mentioned, 36 00:01:57,120 --> 00:01:59,660 can be done in the light or the dark. 37 00:01:59,660 --> 00:02:00,650 They don't need light. 38 00:02:00,650 --> 00:02:03,470 They use the NADPH from the light reactions, 39 00:02:03,470 --> 00:02:06,470 as well as the ATP produced from the light reactions 40 00:02:06,470 --> 00:02:10,580 to take CO2 and turn it into carbohydrate. 41 00:02:10,580 --> 00:02:15,800 And that allows plants to store carbon, reduced carbon energy, 42 00:02:15,800 --> 00:02:18,170 for later, such that when the sun is not 43 00:02:18,170 --> 00:02:21,980 shining they can then use that carbohydrate to oxidize it 44 00:02:21,980 --> 00:02:24,950 in all of the reactions we've already learned about 45 00:02:24,950 --> 00:02:27,440 and make sure they're able to generate ATP, 46 00:02:27,440 --> 00:02:29,960 keep their ATP-ADP ratio high at night, 47 00:02:29,960 --> 00:02:32,180 and then switch back to using photosynthesis when 48 00:02:32,180 --> 00:02:34,520 the sun is out during the day. 49 00:02:34,520 --> 00:02:37,100 Now, today what we're going to discuss 50 00:02:37,100 --> 00:02:42,950 is both the light and the dark reactions in great detail 51 00:02:42,950 --> 00:02:46,440 to fill in how those processes happen. 52 00:02:46,440 --> 00:02:50,490 So first, we're going to start with the light reactions. 53 00:02:50,490 --> 00:02:52,790 And so the light reactions, if it's 54 00:02:52,790 --> 00:02:55,100 like oxidative phosphorylation, that 55 00:02:55,100 --> 00:02:58,220 is we're going to generate a membrane potential, 56 00:02:58,220 --> 00:03:01,680 build a battery, that is to occur at a membrane. 57 00:03:01,680 --> 00:03:07,160 And so those membranes are the cell membrane of a bacteria, 58 00:03:07,160 --> 00:03:25,120 or prokaryote, or an intracellular membrane 59 00:03:25,120 --> 00:03:31,990 of a eukaryotic photosynthetic organism. 60 00:03:31,990 --> 00:03:36,640 And that intracellular membrane is, of course, the chloroplast 61 00:03:36,640 --> 00:03:41,020 that you learned about in grade school. 62 00:03:41,020 --> 00:03:44,260 And so the chloroplast is the photosynthetic equivalent 63 00:03:44,260 --> 00:03:45,790 of the mitochondria. 64 00:03:45,790 --> 00:03:47,980 Oxidative phosphorylation in eukaryotes 65 00:03:47,980 --> 00:03:49,720 happen at a mitochondrial membrane. 66 00:03:49,720 --> 00:03:52,360 And photosynthesis in eukaryotes happens 67 00:03:52,360 --> 00:03:54,800 at a chloroplast membrane. 68 00:03:54,800 --> 00:03:57,370 And so, just to be explicit about this, 69 00:03:57,370 --> 00:04:00,490 so if this is our photosynthetic bacteria, 70 00:04:00,490 --> 00:04:04,960 some prokaryotic cell, it will use photosynthesis 71 00:04:04,960 --> 00:04:10,750 to make delta psi, delta pH across the cell membrane. 72 00:04:10,750 --> 00:04:15,790 And then it can utilize that to provide energy 73 00:04:15,790 --> 00:04:18,730 for the organism, very similar to what 74 00:04:18,730 --> 00:04:22,390 we said about oxidative phosphorylation in a bacteria. 75 00:04:22,390 --> 00:04:24,490 And you can imagine that then there 76 00:04:24,490 --> 00:04:29,680 was some early eukaryotic cell that ended up engulfing 77 00:04:29,680 --> 00:04:34,710 this photosynthetic prokaryote. 78 00:04:34,710 --> 00:04:38,460 And in the process, the ATP generated 79 00:04:38,460 --> 00:04:43,850 could be exchanged for ADP across those membranes. 80 00:04:43,850 --> 00:04:47,090 And in the end, this is how one ends up with this 81 00:04:47,090 --> 00:04:59,490 double-membraned organelle within the engulfed prokaryotic 82 00:04:59,490 --> 00:05:05,400 species, with having the membrane potential built across 83 00:05:05,400 --> 00:05:06,300 this-- 84 00:05:06,300 --> 00:05:08,700 oops-- having the membrane potential 85 00:05:08,700 --> 00:05:12,090 built across this inner mitochondrial membrane-- 86 00:05:12,090 --> 00:05:16,540 or inner membrane of, in this case, the chloroplast. 87 00:05:16,540 --> 00:05:19,480 And so, if you remember, we drew the mitochondria 88 00:05:19,480 --> 00:05:21,635 as having this double membrane structure, 89 00:05:21,635 --> 00:05:23,010 the inner mitochondrial membrane, 90 00:05:23,010 --> 00:05:25,020 where oxidative phosphorylation happen. 91 00:05:25,020 --> 00:05:28,650 Protons were pumped into this intermembrane space. 92 00:05:28,650 --> 00:05:33,570 And chloroplasts have basically the same structure, 93 00:05:33,570 --> 00:05:36,240 but it's a little bit more complicated, 94 00:05:36,240 --> 00:05:37,320 at least on the surface. 95 00:05:37,320 --> 00:05:42,090 And that's because chloroplasts have evolved this further 96 00:05:42,090 --> 00:05:44,850 to have what, essentially, are extreme cristae, 97 00:05:44,850 --> 00:05:47,640 those folds of the mitochondria called cristae. 98 00:05:47,640 --> 00:05:52,440 These extreme folds is basically what happens in a chloroplast. 99 00:05:52,440 --> 00:05:56,790 So if we draw here a giant chloroplast-- 100 00:05:56,790 --> 00:06:00,960 so this here would be the outer membrane of the chloroplast. 101 00:06:00,960 --> 00:06:06,570 So the chloroplast also has an inner membrane. 102 00:06:06,570 --> 00:06:11,130 So this here is then the inner membrane of the chloroplast. 103 00:06:11,130 --> 00:06:18,560 You can imagine that if you had this extreme folding 104 00:06:18,560 --> 00:06:21,350 in of the cristae, but actually pinched off 105 00:06:21,350 --> 00:06:23,750 this little piece of membrane here, you 106 00:06:23,750 --> 00:06:27,890 would now end up with stacks of membrane 107 00:06:27,890 --> 00:06:31,340 within the chloroplast. 108 00:06:31,340 --> 00:06:34,010 You still here have this inner membrane here 109 00:06:34,010 --> 00:06:36,140 that goes all the way around, but now 110 00:06:36,140 --> 00:06:38,840 you have these little membrane stacks that now 111 00:06:38,840 --> 00:06:40,820 exist within the chloroplast. 112 00:06:40,820 --> 00:06:44,690 These here is called grana. 113 00:06:44,690 --> 00:06:52,910 And this inside of these stacks, what is called the lumen, 114 00:06:52,910 --> 00:06:56,660 is really the equivalent of the intermembrane space 115 00:06:56,660 --> 00:06:58,170 of the mitochondria. 116 00:06:58,170 --> 00:07:01,460 So the inside of this is basically equivalent 117 00:07:01,460 --> 00:07:04,700 to the intermembrane space of the mitochondria, 118 00:07:04,700 --> 00:07:10,310 such that if I draw one of these pinched off pieces of membrane 119 00:07:10,310 --> 00:07:15,470 in gigantic form, you're basically pumping protons 120 00:07:15,470 --> 00:07:21,470 into the lumen, into this intermembrane space equivalent. 121 00:07:21,470 --> 00:07:25,970 And so you can imagine that you have this electron transport 122 00:07:25,970 --> 00:07:32,150 chain set up, where electrons are transferred from water 123 00:07:32,150 --> 00:07:37,100 to NADP+, making oxygen an NADPH. 124 00:07:37,100 --> 00:07:40,910 That is, electron transport is made favorable 125 00:07:40,910 --> 00:07:45,920 by light that generates a membrane potential 126 00:07:45,920 --> 00:07:53,830 across this membrane, which is, by the way, this membrane-- 127 00:07:53,830 --> 00:07:55,240 which is the same there-- this is 128 00:07:55,240 --> 00:08:00,790 called the thylakoid membrane. 129 00:08:08,260 --> 00:08:13,670 Thylakoid-- T-H-Y-L-A-K-O-I-D-- membrane. 130 00:08:13,670 --> 00:08:17,140 So you build a potential across this thylakoid membrane, 131 00:08:17,140 --> 00:08:24,040 and then that potential can be used 132 00:08:24,040 --> 00:08:32,820 to synthesize ATP within this other space, 133 00:08:32,820 --> 00:08:39,039 this wider space here, which is called the stroma, which 134 00:08:39,039 --> 00:08:44,500 is the equivalent of the matrix of the mitochondria. 135 00:08:44,500 --> 00:08:49,120 And so basically, the chloroplast 136 00:08:49,120 --> 00:08:52,210 is set up in a very similar way to the mitochondria, 137 00:08:52,210 --> 00:08:55,180 but because it makes these membrane stacks, 138 00:08:55,180 --> 00:08:57,170 it looks like it's a little bit different. 139 00:08:57,170 --> 00:09:01,540 But it's really pumping protons into the intermembrane space, 140 00:09:01,540 --> 00:09:04,300 which would happen in the mitochondria an equivalent 141 00:09:04,300 --> 00:09:07,780 space, which is the lumen of these stacks, 142 00:09:07,780 --> 00:09:09,850 is what happens within the chloroplast. 143 00:09:09,850 --> 00:09:13,810 That charge on the membrane, that delta psi, delta pH, 144 00:09:13,810 --> 00:09:17,650 can then be used to do work like synthesizing ATP. 145 00:09:17,650 --> 00:09:20,740 That happens in-- rather than-- 146 00:09:20,740 --> 00:09:26,500 and that happens in the stroma of the matrix equivalent, 147 00:09:26,500 --> 00:09:28,580 the stroma of the chloroplast. 148 00:09:28,580 --> 00:09:31,330 And so effectively, you have NADPH, 149 00:09:31,330 --> 00:09:33,880 an ATP produced in the stroma. 150 00:09:33,880 --> 00:09:38,560 And so it turns out the dark reactions, when 151 00:09:38,560 --> 00:09:41,740 we talk about them later, will occur here 152 00:09:41,740 --> 00:09:44,470 in the stroma of the chloroplast. 153 00:09:44,470 --> 00:09:47,170 And the light reactions, of course, 154 00:09:47,170 --> 00:09:53,125 are happening over here at the thylakoid membrane. 155 00:09:55,930 --> 00:09:59,590 Now, this is actually a cool innovation 156 00:09:59,590 --> 00:10:02,060 that nature has come up with for this. 157 00:10:02,060 --> 00:10:05,230 And because by making these extreme cristae, 158 00:10:05,230 --> 00:10:07,750 you can imagine that there's a very small space 159 00:10:07,750 --> 00:10:10,000 here that you're pumping protons into. 160 00:10:10,000 --> 00:10:14,080 That means that fewer protons pump will allow you to generate 161 00:10:14,080 --> 00:10:21,640 a higher potential, a higher gradient of protons, delta pH. 162 00:10:21,640 --> 00:10:27,190 And so that means that that then can be used to drive energy 163 00:10:27,190 --> 00:10:29,620 in a more efficient way then you might 164 00:10:29,620 --> 00:10:36,430 get using this much larger intermembrane space. 165 00:10:36,430 --> 00:10:39,670 And so light reactions here at the thylakoid membrane 166 00:10:39,670 --> 00:10:42,040 within the chloroplast, dark reactions 167 00:10:42,040 --> 00:10:45,820 here in the stroma of the chloroplast. 168 00:10:45,820 --> 00:10:47,140 All right. 169 00:10:47,140 --> 00:10:49,210 So that's the anatomy. 170 00:10:49,210 --> 00:10:53,020 Now, let's get back to what's actually happening here 171 00:10:53,020 --> 00:10:56,160 in these light reactions. 172 00:10:56,160 --> 00:10:58,640 So of course, the trick of the light reactions 173 00:10:58,640 --> 00:11:03,230 is to come up with some way to make water a good electron 174 00:11:03,230 --> 00:11:06,680 donor, such that you can have favorable electron 175 00:11:06,680 --> 00:11:10,850 transfer to NADPH, which is, of course, what's 176 00:11:10,850 --> 00:11:15,470 necessary to have energy released to generate delta psi, 177 00:11:15,470 --> 00:11:16,910 delta pH. 178 00:11:16,910 --> 00:11:19,010 And so, of course, to do this, we still 179 00:11:19,010 --> 00:11:22,310 have to follow all the same thermodynamic rules that 180 00:11:22,310 --> 00:11:25,460 existed before, that we talked about 181 00:11:25,460 --> 00:11:27,480 for oxidative phosphorylation. 182 00:11:27,480 --> 00:11:29,270 So you should remember from those lectures 183 00:11:29,270 --> 00:11:31,940 that electron transfer is going to be favorable 184 00:11:31,940 --> 00:11:35,720 if we move from a lower to a higher standard 185 00:11:35,720 --> 00:11:37,580 reduction potential. 186 00:11:37,580 --> 00:11:39,230 So if we just draw this out. 187 00:11:39,230 --> 00:11:44,210 So here's a physiological range of standard reduction 188 00:11:44,210 --> 00:11:52,890 potentials from negative 1.6 to positive 0.8 volts. 189 00:11:52,890 --> 00:11:58,890 So this here would be a range of standard reduction potentials. 190 00:11:58,890 --> 00:12:02,360 So remember, a change in standard reduction potential 191 00:12:02,360 --> 00:12:07,310 that's positive, that means moving this direction down, 192 00:12:07,310 --> 00:12:11,870 as I've drawn this axis from smaller to larger. 193 00:12:11,870 --> 00:12:14,990 I realize I drew it upside down in the way you probably 194 00:12:14,990 --> 00:12:16,670 are normally used to looking at this. 195 00:12:16,670 --> 00:12:18,170 But basically, if you go from a more 196 00:12:18,170 --> 00:12:20,480 negative to a positive number, or a more negative 197 00:12:20,480 --> 00:12:23,150 to a less negative, or a positive to a more positive, 198 00:12:23,150 --> 00:12:24,930 basically, a change in this direction, 199 00:12:24,930 --> 00:12:27,470 which is going to have a positive change 200 00:12:27,470 --> 00:12:29,330 in standard reduction potential, is 201 00:12:29,330 --> 00:12:32,840 going to end up having a negative change in delta g-- 202 00:12:32,840 --> 00:12:33,920 not prime. 203 00:12:33,920 --> 00:12:35,900 And so that means equilibrium is going 204 00:12:35,900 --> 00:12:40,590 to favor electron transport moving down in this direction. 205 00:12:40,590 --> 00:12:44,550 So now, let's just give this some real numbers. 206 00:12:44,550 --> 00:12:50,870 So the standard reduction potential of oxygen water 207 00:12:50,870 --> 00:12:52,290 sits down here. 208 00:12:52,290 --> 00:12:54,400 It's 0.86. 209 00:12:54,400 --> 00:12:57,770 You don't have to worry about exact numbers. 210 00:12:57,770 --> 00:13:09,282 The standard reduction potential of NADH, or NADPH, 211 00:13:09,282 --> 00:13:09,990 they're the same. 212 00:13:09,990 --> 00:13:12,060 The nicotinamide group is here. 213 00:13:12,060 --> 00:13:15,990 It's about negative 0.32. 214 00:13:15,990 --> 00:13:21,300 And this is why oxidative phosphorylation works 215 00:13:21,300 --> 00:13:24,900 is because there is a net positive change 216 00:13:24,900 --> 00:13:26,280 in standard reduction potential. 217 00:13:26,280 --> 00:13:28,540 That is, you go from a lower to a higher standard 218 00:13:28,540 --> 00:13:31,470 reduction potential if we do electron 219 00:13:31,470 --> 00:13:35,880 transport in the oxidative phosphorylation. 220 00:13:35,880 --> 00:13:38,640 That means energy is released as we transfer electrons 221 00:13:38,640 --> 00:13:44,700 from NADH to oxygen. So that is favorable. 222 00:13:44,700 --> 00:13:47,220 That favorable energy release is what 223 00:13:47,220 --> 00:13:49,590 can be coupled to proton pumping to generate 224 00:13:49,590 --> 00:13:53,550 delta psi, delta pH, and allow the cell to do other work. 225 00:13:53,550 --> 00:14:01,360 Now, you cannot change the properties of water just 226 00:14:01,360 --> 00:14:04,270 because you add light to it. 227 00:14:04,270 --> 00:14:07,840 So it still sits at this standard reduction potential. 228 00:14:07,840 --> 00:14:10,240 But what happens in photosynthesis 229 00:14:10,240 --> 00:14:16,480 is basically you use light to excite an electron. 230 00:14:16,480 --> 00:14:21,760 That effectively moves water up here 231 00:14:21,760 --> 00:14:26,050 to a more negative electron up here, 232 00:14:26,050 --> 00:14:27,910 to generate an electron that's up here 233 00:14:27,910 --> 00:14:30,280 at a more negative standard reduction 234 00:14:30,280 --> 00:14:35,710 potential than NADPH, which, of course, now moving down, 235 00:14:35,710 --> 00:14:37,960 becomes favorable. 236 00:14:37,960 --> 00:14:41,900 And so here, now you're going from more negative to here. 237 00:14:41,900 --> 00:14:45,430 And then from there to there, each of those is favorable. 238 00:14:45,430 --> 00:14:48,310 This means you can use this favorable electron 239 00:14:48,310 --> 00:14:52,030 transfer, the energy release from the favorable movement 240 00:14:52,030 --> 00:14:56,920 of electrons from this excited electron, to NADPH, 241 00:14:56,920 --> 00:14:58,360 generate NADPH. 242 00:14:58,360 --> 00:14:59,920 And then you can also use-- 243 00:14:59,920 --> 00:15:02,710 donate those to the electron transport chain 244 00:15:02,710 --> 00:15:04,900 and OXPHOS with oxygen-- 245 00:15:04,900 --> 00:15:08,290 with water-- with oxygen as the final electron acceptor to net 246 00:15:08,290 --> 00:15:11,630 generate delta psi, delta pH in OXPHOS. 247 00:15:11,630 --> 00:15:15,010 And so this works to generate it in photosynthesis, 248 00:15:15,010 --> 00:15:19,570 and this works to generate it in oxidative phosphorylation. 249 00:15:19,570 --> 00:15:22,360 And we've not violated any laws of thermodynamics. 250 00:15:22,360 --> 00:15:26,170 And we're able to, in both cases, 251 00:15:26,170 --> 00:15:27,910 create this membrane potential, build 252 00:15:27,910 --> 00:15:29,920 this battery, that now allows you 253 00:15:29,920 --> 00:15:34,960 to do work such as synthesize ATP, 254 00:15:34,960 --> 00:15:37,690 keep the ATP-ADP ratio high, allow the cell 255 00:15:37,690 --> 00:15:42,960 to do all the unfavorable reactions that it needs to do. 256 00:15:42,960 --> 00:15:47,710 So now, let's talk a bit about this process. 257 00:15:47,710 --> 00:15:50,640 So of course, this happens with visible light, 258 00:15:50,640 --> 00:15:54,570 but I want to start with asking a fundamental question. 259 00:15:54,570 --> 00:15:56,910 Why do we use visible light? 260 00:15:56,910 --> 00:16:01,320 Or, to put it another way, what is visible light, anyway? 261 00:16:01,320 --> 00:16:04,490 Well, visible light is, of course, just 262 00:16:04,490 --> 00:16:06,770 electromagnetic radiation, a photon 263 00:16:06,770 --> 00:16:09,740 on the electromagnetic radiation spectrum. 264 00:16:09,740 --> 00:16:12,860 And visible light is defined by what we see. 265 00:16:12,860 --> 00:16:15,730 So if we draw this out-- 266 00:16:15,730 --> 00:16:19,340 this is hopefully something that you've 267 00:16:19,340 --> 00:16:23,860 covered in a physics class somewhere along the way. 268 00:16:23,860 --> 00:16:31,360 So if this here is the electromagnetic magnetic 269 00:16:31,360 --> 00:16:35,530 spectrum as drawn out-- so down here you have your short 270 00:16:35,530 --> 00:16:38,800 wavelength electromagnetic radiation, 271 00:16:38,800 --> 00:16:45,800 down here you have your long wavelength things-- 272 00:16:45,800 --> 00:16:49,280 so higher energy, lower energy-- 273 00:16:49,280 --> 00:16:52,220 and on this end, X-ray, UV light. 274 00:16:52,220 --> 00:16:54,560 And you get to the visible spectrum 275 00:16:54,560 --> 00:17:00,710 and we go from the purple light down to the red light. 276 00:17:00,710 --> 00:17:03,500 And eventually, we get into the infrared light, 277 00:17:03,500 --> 00:17:06,740 and ultimately, down to microwaves, 278 00:17:06,740 --> 00:17:11,609 and basically shorter wavelength and longer length wavelengths, 279 00:17:11,609 --> 00:17:14,560 more energy, less energy. 280 00:17:14,560 --> 00:17:19,650 And so just to give you some numbers, so this here 281 00:17:19,650 --> 00:17:22,300 is on the order of 300 kilojoules, 282 00:17:22,300 --> 00:17:25,270 is on the order of 170 kilojoules. 283 00:17:25,270 --> 00:17:30,960 And so ADP to ATP conversion-- so we before 284 00:17:30,960 --> 00:17:32,670 talked about in kcals per mole, but it's 285 00:17:32,670 --> 00:17:36,550 on the order of 30 kilojoules for that. 286 00:17:36,550 --> 00:17:41,130 And so this part of the electromagnetic spectrum 287 00:17:41,130 --> 00:17:43,620 is the right order of magnitude, it 288 00:17:43,620 --> 00:17:48,480 turns out, to transduce energy in a realm that 289 00:17:48,480 --> 00:17:53,400 matches the energy required, or release, from the ATP 290 00:17:53,400 --> 00:17:57,180 to ADP conversion reaction. 291 00:17:57,180 --> 00:18:01,500 You also know from just popular use, 292 00:18:01,500 --> 00:18:03,630 you go out in the sun when it's a nice day, 293 00:18:03,630 --> 00:18:04,800 you put sunscreen on. 294 00:18:04,800 --> 00:18:05,920 Why do you do that? 295 00:18:05,920 --> 00:18:08,730 That's because the UV light that's 296 00:18:08,730 --> 00:18:10,830 filtered through our atmosphere from the sun 297 00:18:10,830 --> 00:18:12,120 can cause sunburns. 298 00:18:12,120 --> 00:18:15,480 That is, it can cause damage to our skin, 299 00:18:15,480 --> 00:18:17,760 to other biological molecules. 300 00:18:17,760 --> 00:18:20,700 Obviously, X-rays, you go get an X-ray done, 301 00:18:20,700 --> 00:18:22,950 you get covered with lead in all the places they don't 302 00:18:22,950 --> 00:18:24,610 want to have the X-ray done. 303 00:18:24,610 --> 00:18:25,110 Why? 304 00:18:25,110 --> 00:18:29,220 Because X-rays can cause damage. 305 00:18:29,220 --> 00:18:31,860 We use those to treat cancer. 306 00:18:31,860 --> 00:18:36,060 And so things down on this end, too much energy damages 307 00:18:36,060 --> 00:18:37,950 biological molecules. 308 00:18:37,950 --> 00:18:41,370 This is the sweet spot where you don't get a lot of damage 309 00:18:41,370 --> 00:18:42,900 to the biological molecules. 310 00:18:42,900 --> 00:18:45,300 And the energy is right to synthesize-- 311 00:18:47,980 --> 00:18:51,250 to transduce energy on a magnitude that 312 00:18:51,250 --> 00:18:53,140 works for biological systems. 313 00:18:53,140 --> 00:18:54,790 Come down to this end, well, now you 314 00:18:54,790 --> 00:18:56,440 get to be too little energy. 315 00:18:56,440 --> 00:18:59,380 You get down to the infrared and the microwaves. 316 00:18:59,380 --> 00:19:01,850 This is like your TV remote control. 317 00:19:01,850 --> 00:19:05,577 This is the cell phones that if you were in class as opposed 318 00:19:05,577 --> 00:19:07,660 to listening to this online, you would be checking 319 00:19:07,660 --> 00:19:09,130 instead of listening to me. 320 00:19:09,130 --> 00:19:13,210 So this direction, too little energy 321 00:19:13,210 --> 00:19:17,260 is transduced to actually matter for the biological systems. 322 00:19:17,260 --> 00:19:21,610 And so that's really why this spectrum of light 323 00:19:21,610 --> 00:19:24,190 is what's involved in photosynthesis. 324 00:19:24,190 --> 00:19:26,860 However, I also want to remind you 325 00:19:26,860 --> 00:19:29,090 that visible light is a human construct. 326 00:19:29,090 --> 00:19:31,480 It's what we as humans can see. 327 00:19:31,480 --> 00:19:35,620 And photosynthesis evolved long before humans. 328 00:19:35,620 --> 00:19:39,010 We needed photosynthesis first before any animal life 329 00:19:39,010 --> 00:19:40,480 can evolve. 330 00:19:40,480 --> 00:19:44,140 And so this visible light designation 331 00:19:44,140 --> 00:19:46,660 is completely arbitrary. 332 00:19:46,660 --> 00:19:50,890 And in fact, we evolved from photosynthetic organisms. 333 00:19:50,890 --> 00:19:53,350 And so it's really photosynthetic pigments 334 00:19:53,350 --> 00:19:57,520 that evolved first to capture light in this range 335 00:19:57,520 --> 00:20:01,540 that we then repurposed in order to see. 336 00:20:01,540 --> 00:20:06,880 So in essence, photosynthesis defines the visual spectrum, 337 00:20:06,880 --> 00:20:10,270 because what we see is defined by the pigments that 338 00:20:10,270 --> 00:20:12,190 came-- because it evolved from photosynthesis. 339 00:20:12,190 --> 00:20:14,760 The carotenoids, the carrot, the why 340 00:20:14,760 --> 00:20:16,510 do eat carrots that are good for our eyes? 341 00:20:16,510 --> 00:20:19,780 They have the pigments in carrots, carotenoids-- 342 00:20:19,780 --> 00:20:21,490 which we'll draw in a little bit-- 343 00:20:21,490 --> 00:20:26,080 that are ultimately related to our visual pigments. 344 00:20:26,080 --> 00:20:27,430 And we didn't invent those. 345 00:20:27,430 --> 00:20:29,170 Photosynthetic organisms invented 346 00:20:29,170 --> 00:20:33,520 those to work in this spectrum to harvest light 347 00:20:33,520 --> 00:20:34,840 from the sun for energy. 348 00:20:34,840 --> 00:20:38,410 And we repurposed those molecules as a way for us 349 00:20:38,410 --> 00:20:41,680 to see. 350 00:20:41,680 --> 00:20:44,370 So how does this work? 351 00:20:44,370 --> 00:20:48,020 Well, again, I got to remind you of a little bit more physics, 352 00:20:48,020 --> 00:20:49,460 a little quantum physics. 353 00:20:49,460 --> 00:20:51,620 This is not-- obviously, we're not 354 00:20:51,620 --> 00:20:53,378 going to go into this at the level you 355 00:20:53,378 --> 00:20:54,920 get in a physics class, and hopefully 356 00:20:54,920 --> 00:20:56,570 you've seen this before. 357 00:20:56,570 --> 00:20:58,370 But you remember, if you think back 358 00:20:58,370 --> 00:21:01,280 to physics and physical chemistry, 359 00:21:01,280 --> 00:21:06,410 that electrons sit in discrete orbitals within molecules. 360 00:21:06,410 --> 00:21:09,300 So we can draw those orbitals like this. 361 00:21:09,300 --> 00:21:13,400 And so you'd have some electron pair sitting there 362 00:21:13,400 --> 00:21:16,830 in some orbital in a molecule. 363 00:21:16,830 --> 00:21:22,830 And if you deliver light with the right amount of energy, 364 00:21:22,830 --> 00:21:26,510 the right quanta of energy, you can now 365 00:21:26,510 --> 00:21:32,420 excite one of those electrons up here into a orbital 366 00:21:32,420 --> 00:21:34,970 with a higher energy. 367 00:21:34,970 --> 00:21:40,230 And of course, that's not stable, so it will decay. 368 00:21:40,230 --> 00:21:47,000 And when that electron decays back to the ground state, 369 00:21:47,000 --> 00:21:52,720 this then re-releases another photon of light 370 00:21:52,720 --> 00:21:55,300 at a longer wavelength. 371 00:21:55,300 --> 00:21:58,270 So longer wavelength light is emitted, 372 00:21:58,270 --> 00:22:00,920 and that's effectively fluorescence. 373 00:22:00,920 --> 00:22:03,730 So you excite something, electron bounces up 374 00:22:03,730 --> 00:22:05,830 into a higher orbital, it decays, 375 00:22:05,830 --> 00:22:09,610 releases some of the energy that it absorbed 376 00:22:09,610 --> 00:22:11,710 as it transitions back. 377 00:22:11,710 --> 00:22:13,180 That is fluorescence. 378 00:22:13,180 --> 00:22:14,800 This happens very, very fast. 379 00:22:14,800 --> 00:22:16,600 And basically what photosynthesis 380 00:22:16,600 --> 00:22:20,860 does is it takes advantage of this excited state. 381 00:22:20,860 --> 00:22:24,970 And it effectively transfers that electron away, 382 00:22:24,970 --> 00:22:27,820 creating a charged separation prior 383 00:22:27,820 --> 00:22:34,900 to that electron decaying back into the ground state orbital. 384 00:22:34,900 --> 00:22:36,400 This occurs very fast. 385 00:22:36,400 --> 00:22:39,880 And so photosynthesis has to occur very, very fast in order 386 00:22:39,880 --> 00:22:41,680 to make this work. 387 00:22:41,680 --> 00:22:46,300 And in essence, this is the way that photosynthesis 388 00:22:46,300 --> 00:22:49,780 is able to capture energy and connect it 389 00:22:49,780 --> 00:22:52,600 to a biological system that allows us to build 390 00:22:52,600 --> 00:22:54,560 an electron transport chain. 391 00:22:54,560 --> 00:22:57,490 And so in essence, photosynthesis, the way 392 00:22:57,490 --> 00:23:01,270 the energy is absorbed, is it's what you learned in physics. 393 00:23:01,270 --> 00:23:03,610 The right quanta of light causes an electron 394 00:23:03,610 --> 00:23:06,070 to excite into a higher energy orbital. 395 00:23:06,070 --> 00:23:08,170 And then that electron is transferred away, 396 00:23:08,170 --> 00:23:10,780 creating a charged separation before 397 00:23:10,780 --> 00:23:13,570 that energy of that excited electron 398 00:23:13,570 --> 00:23:15,610 can decay as fluorescence. 399 00:23:15,610 --> 00:23:18,040 That creates a charged separation. 400 00:23:18,040 --> 00:23:22,990 And that charged separation now has an electron such 401 00:23:22,990 --> 00:23:28,330 that that electron can now be favorably transferred to NADPH. 402 00:23:28,330 --> 00:23:31,430 That energy released in that favorable transfer 403 00:23:31,430 --> 00:23:34,060 can be coupled to make delta psi, delta pH. 404 00:23:34,060 --> 00:23:35,810 And then that translates as energy 405 00:23:35,810 --> 00:23:37,900 in exactly the same way we learned about 406 00:23:37,900 --> 00:23:42,100 for oxidative phosphorylation. 407 00:23:42,100 --> 00:23:45,040 So to do this, what do we need? 408 00:23:45,040 --> 00:23:49,930 Well, we need a pigment that can absorb a photon of the right 409 00:23:49,930 --> 00:23:53,350 wavelength-- or, put it another way, of the right energy-- 410 00:23:53,350 --> 00:23:58,120 and then quickly transfer that electron away. 411 00:23:58,120 --> 00:23:59,330 And so you need two things. 412 00:23:59,330 --> 00:24:01,930 You need light acceptors that can 413 00:24:01,930 --> 00:24:04,370 absorb energy of the right energy, 414 00:24:04,370 --> 00:24:07,900 the right wavelength of light, and redox carriers to transfer 415 00:24:07,900 --> 00:24:09,340 those electrons away. 416 00:24:09,340 --> 00:24:11,470 So what are the light acceptors? 417 00:24:11,470 --> 00:24:13,510 Well, these are basically pigments 418 00:24:13,510 --> 00:24:15,910 that can absorb visible light. 419 00:24:15,910 --> 00:24:18,130 And redox carriers are electron carriers, 420 00:24:18,130 --> 00:24:21,860 which we've been talking about now for several lectures. 421 00:24:21,860 --> 00:24:23,690 So let's start with the light acceptor. 422 00:24:23,690 --> 00:24:26,090 So this is something that's tuned to the right energy, 423 00:24:26,090 --> 00:24:27,200 the right wavelength. 424 00:24:27,200 --> 00:24:30,680 And so this is chlorophyll. 425 00:24:35,090 --> 00:24:37,280 So we all know plants contain chlorophyll. 426 00:24:37,280 --> 00:24:39,260 That's important for photosynthesis. 427 00:24:39,260 --> 00:24:41,180 Well, what is chlorophyll? 428 00:24:41,180 --> 00:24:44,120 Well, it's a conjugated pyrrole, very similar 429 00:24:44,120 --> 00:24:46,955 to heme, except remember, heme had iron in it. 430 00:24:46,955 --> 00:24:49,580 Saw that for hemoglobin, saw it again in the electron transport 431 00:24:49,580 --> 00:24:50,285 chain. 432 00:24:50,285 --> 00:24:51,620 And I'm going to show you chlorophyll. 433 00:24:51,620 --> 00:24:53,495 Chlorophyll is going to look a lot like that, 434 00:24:53,495 --> 00:24:58,610 except it has a magnesium conjugated to the tetrapyrrole 435 00:24:58,610 --> 00:25:00,950 rather than iron. 436 00:25:00,950 --> 00:25:36,080 And so here you have tetrapyrrole 437 00:25:36,080 --> 00:25:39,070 with a magnesium in the middle, conjugated double bond 438 00:25:39,070 --> 00:25:41,170 system all the way around. 439 00:25:41,170 --> 00:25:44,740 Makes sense, that should absorb visible light, 440 00:25:44,740 --> 00:25:47,620 all those conjugated double bonds. 441 00:25:47,620 --> 00:25:52,180 There's some R group decorations hanging off over here. 442 00:25:52,180 --> 00:25:57,730 Chlorophyll also has this additional decoration 443 00:25:57,730 --> 00:26:11,370 over here, including a long lipid tail here to make this-- 444 00:26:11,370 --> 00:26:12,955 it will sit within a membrane. 445 00:26:18,788 --> 00:26:19,288 Oops. 446 00:26:26,290 --> 00:26:27,060 Apologies. 447 00:26:27,060 --> 00:26:27,790 I misdrew that. 448 00:26:41,450 --> 00:26:43,640 Lipid tail hangs off down there. 449 00:26:43,640 --> 00:26:46,040 But this here is effectively what 450 00:26:46,040 --> 00:26:49,550 the structure of chlorophyll looks like, 451 00:26:49,550 --> 00:26:54,608 with this R and R prime being specific groups. 452 00:26:54,608 --> 00:26:56,150 You can look up if you're interested. 453 00:26:56,150 --> 00:27:00,290 But basically, these are the decorations 454 00:27:00,290 --> 00:27:08,120 that define what I refer to as A type and B type chlorophylls. 455 00:27:08,120 --> 00:27:11,090 Remember, we had A, B, and C type cytochromes that 456 00:27:11,090 --> 00:27:14,030 were basically similar molecules with slightly different 457 00:27:14,030 --> 00:27:17,150 decorations on them, the types defined slightly 458 00:27:17,150 --> 00:27:21,260 by the absorbance properties of these pigments. 459 00:27:21,260 --> 00:27:23,540 But effectively, what's key about chlorophyll 460 00:27:23,540 --> 00:27:25,460 is that it's this conjugated, double bond 461 00:27:25,460 --> 00:27:29,630 system in the tetrapyrrole that ultimately allows 462 00:27:29,630 --> 00:27:33,080 it to absorb visible light. 463 00:27:33,080 --> 00:27:35,810 Now, chlorophyll is, of course, the most famous pigment, 464 00:27:35,810 --> 00:27:38,930 but it's not the only pigment that photosynthetic organisms 465 00:27:38,930 --> 00:27:39,740 used. 466 00:27:39,740 --> 00:27:44,820 So they also use other pigments to absorb visible light. 467 00:27:44,820 --> 00:27:47,660 And so there is a class of pigments that look 468 00:27:47,660 --> 00:27:49,910 very similar to chlorophyll. 469 00:27:49,910 --> 00:27:52,200 You can look up the structures if you want. 470 00:27:52,200 --> 00:27:53,630 They're called phytobilins. 471 00:27:53,630 --> 00:27:58,610 They're also tetrapyrroles, very similar to chlorophyll, 472 00:27:58,610 --> 00:28:05,690 except, for instance, pheophytin does not 473 00:28:05,690 --> 00:28:10,820 contain any magnesium or other thing conjugated in the center, 474 00:28:10,820 --> 00:28:13,430 so no magnesium. 475 00:28:13,430 --> 00:28:15,503 Otherwise, it looks similar to chlorophyll. 476 00:28:15,503 --> 00:28:16,670 And then there's other ones. 477 00:28:16,670 --> 00:28:28,930 There's so-called phycoerythrin, which you might guess is red. 478 00:28:28,930 --> 00:28:30,540 So that's a red pigment. 479 00:28:30,540 --> 00:28:37,670 There's phycocyanin, which is, you might guess, 480 00:28:37,670 --> 00:28:43,400 is a blue pigment. 481 00:28:43,400 --> 00:28:49,910 And then maybe the most famous one that about that's 482 00:28:49,910 --> 00:28:53,570 not chlorophyll are the so-called carotenoids. 483 00:28:53,570 --> 00:28:56,950 This includes things like beta-carotene. 484 00:28:59,690 --> 00:29:03,110 I think we all know that beta-carotene is orange. 485 00:29:06,590 --> 00:29:08,750 What does beta-carotene look like? 486 00:29:08,750 --> 00:29:10,870 I'll draw it out for you. 487 00:29:39,290 --> 00:29:43,760 There's beta-carotene, another long conjugated, double bond 488 00:29:43,760 --> 00:29:44,528 system. 489 00:29:44,528 --> 00:29:46,070 Obviously, you don't need to memorize 490 00:29:46,070 --> 00:29:48,050 the structures of any of these. 491 00:29:48,050 --> 00:29:50,750 You can always look up structures of these pigments. 492 00:29:50,750 --> 00:29:56,780 The point is that nature has a variety of these structures, 493 00:29:56,780 --> 00:29:58,940 all with conjugated double bond systems, 494 00:29:58,940 --> 00:30:03,050 all well-tuned to absorb visible light. 495 00:30:03,050 --> 00:30:08,510 And this, of course, also then becomes a major pigment 496 00:30:08,510 --> 00:30:12,560 for our visual system, repurposed 497 00:30:12,560 --> 00:30:16,040 from the plant using it for chlorosynthesis-- 498 00:30:16,040 --> 00:30:17,870 for photosynthesis. 499 00:30:17,870 --> 00:30:19,580 And why have all these pigments? 500 00:30:19,580 --> 00:30:22,970 Well, the idea is because if we say here, 501 00:30:22,970 --> 00:30:26,960 what's the absorbance then across the electromagnetic 502 00:30:26,960 --> 00:30:30,530 spectrum, that the purple, the short wavelength, 503 00:30:30,530 --> 00:30:34,160 being down here, and the red, the long wavelength, 504 00:30:34,160 --> 00:30:36,860 being on this end, well now, effectively, 505 00:30:36,860 --> 00:30:42,770 you have chlorophylls that cover that spectrum. 506 00:30:42,770 --> 00:30:48,410 And so you'll have some peak here 507 00:30:48,410 --> 00:30:55,610 for say A type chlorophylls. 508 00:30:55,610 --> 00:31:01,610 And then you'll have some other peak for B type chlorophylls. 509 00:31:04,190 --> 00:31:05,900 These are obviously approximate. 510 00:31:05,900 --> 00:31:09,650 And so that's basically the A and B type chlorophylls. 511 00:31:09,650 --> 00:31:10,820 Why are plants green? 512 00:31:10,820 --> 00:31:14,630 Because they absorb in two peaks on either end, and so that 513 00:31:14,630 --> 00:31:17,720 reflects the green light in the middle. 514 00:31:17,720 --> 00:31:19,400 But now you have these other pigments 515 00:31:19,400 --> 00:31:20,700 that will fill in the middle. 516 00:31:20,700 --> 00:31:23,960 And so the beta-carotene, the orange beta-carotene, 517 00:31:23,960 --> 00:31:25,055 will allow you to-- 518 00:31:30,420 --> 00:31:32,640 plants, they'll have an absorption spectrum that will 519 00:31:32,640 --> 00:31:35,500 peak somewhere in that range. 520 00:31:35,500 --> 00:31:38,250 And then you'll have the phycoerythrins 521 00:31:38,250 --> 00:31:42,750 with an absorption spectrum that will peak somewhere 522 00:31:42,750 --> 00:31:44,200 in that range. 523 00:31:44,200 --> 00:31:48,270 And then the phycocyanins with an absorption spectrum 524 00:31:48,270 --> 00:31:50,680 that will peak somewhere in that range. 525 00:31:50,680 --> 00:31:54,900 And basically, by covering all of these different pigments, 526 00:31:54,900 --> 00:31:57,510 plants and other photosynthetic organisms 527 00:31:57,510 --> 00:32:00,780 can now adapt to different light niches 528 00:32:00,780 --> 00:32:06,240 within the natural world, really capturing the full spectrum 529 00:32:06,240 --> 00:32:07,710 of visible light. 530 00:32:07,710 --> 00:32:09,600 And the nice thing is this also leads 531 00:32:09,600 --> 00:32:12,570 to all the beautiful colors and whatnot 532 00:32:12,570 --> 00:32:15,720 that exists across photosynthetic organisms, 533 00:32:15,720 --> 00:32:18,930 different plants, algaes, and bacteria, all different colors. 534 00:32:18,930 --> 00:32:22,950 And basically will use these different pigments 535 00:32:22,950 --> 00:32:27,960 to ultimately cover this visual spectrum, this right spectrum 536 00:32:27,960 --> 00:32:31,410 with the right amount of energy to carry out 537 00:32:31,410 --> 00:32:35,910 the processes that's necessary to run the light reactions 538 00:32:35,910 --> 00:32:39,000 of photosynthesis. 539 00:32:39,000 --> 00:32:44,520 Now, even though there's this variety of pigments, 540 00:32:44,520 --> 00:32:49,035 effectively, all photosynthetic organisms, as far as we know, 541 00:32:49,035 --> 00:32:52,830 work in a very similar way, in that they basically 542 00:32:52,830 --> 00:32:56,940 arrange these pigments always at a membrane, 543 00:32:56,940 --> 00:33:01,650 always here at this thylakoid membrane in the chloroplast-- 544 00:33:01,650 --> 00:33:05,730 or, I guess it would be the cell membrane of a prokaryote. 545 00:33:05,730 --> 00:33:09,840 And these are organized into a structure called 546 00:33:09,840 --> 00:33:11,985 the light harvesting complex. 547 00:33:18,840 --> 00:33:21,660 And basically, what a light harvesting complex 548 00:33:21,660 --> 00:33:27,000 is, is it's pigments arranged in a structure that basically 549 00:33:27,000 --> 00:33:31,590 can channel energy from the photons captured down 550 00:33:31,590 --> 00:33:35,780 into a place called the reaction center. 551 00:33:39,150 --> 00:33:42,870 So you can imagine that you'd have all kinds of pigments 552 00:33:42,870 --> 00:33:48,300 arranged in a way that basically will absorb photons, release 553 00:33:48,300 --> 00:33:51,690 photons as fluorescence, ultimately going down 554 00:33:51,690 --> 00:33:54,390 to this reaction center. 555 00:33:54,390 --> 00:33:57,810 And it's in the reaction center that really the magic 556 00:33:57,810 --> 00:34:00,880 of photosynthesis happens. 557 00:34:00,880 --> 00:34:04,740 So what happens at the reaction center 558 00:34:04,740 --> 00:34:08,280 is pretty well understood from the structure 559 00:34:08,280 --> 00:34:10,920 of the photosynthetic reaction center that 560 00:34:10,920 --> 00:34:18,600 was solved about 30 years ago in the purple photosynthetic 561 00:34:18,600 --> 00:34:19,650 bacteria. 562 00:34:19,650 --> 00:34:22,570 And this is very illustrative for how it works. 563 00:34:22,570 --> 00:34:25,500 And so if you look here at the slide, 564 00:34:25,500 --> 00:34:28,920 this here is basically a graphical representation 565 00:34:28,920 --> 00:34:32,230 of what that reaction center looks like. 566 00:34:32,230 --> 00:34:35,880 And so effectively, you have a bunch 567 00:34:35,880 --> 00:34:39,600 of, in this particular case, molecules up here. 568 00:34:39,600 --> 00:34:43,770 But really, the reaction center is what exists down here. 569 00:34:43,770 --> 00:34:46,642 And in there, there is a pair of chlorophylls-- 570 00:34:46,642 --> 00:34:47,850 we'll talk about in a second. 571 00:34:47,850 --> 00:34:50,460 Those are they're shown graphically over here. 572 00:34:50,460 --> 00:34:53,610 And they will effectively transfer electrons 573 00:34:53,610 --> 00:34:59,860 around in this direction to these quinones further away 574 00:34:59,860 --> 00:35:03,020 from the special pair of chlorophyll. 575 00:35:03,020 --> 00:35:05,290 So electrons will be transferred from the chlorophyll 576 00:35:05,290 --> 00:35:07,720 after it's excited down to these quinones, 577 00:35:07,720 --> 00:35:11,860 creating a charged separation that ultimately is going 578 00:35:11,860 --> 00:35:15,100 to allow you to then have a high-energy electron that 579 00:35:15,100 --> 00:35:18,620 can be built into an electron transport chain. 580 00:35:18,620 --> 00:35:22,850 And so let's draw that graphically out over here. 581 00:35:22,850 --> 00:35:28,640 And so here if we have our special pair of chlorophylls-- 582 00:35:28,640 --> 00:35:30,970 so chlorophyll, if you look at it here, 583 00:35:30,970 --> 00:35:32,650 is relatively a pyrrole. 584 00:35:32,650 --> 00:35:35,770 Like heme, it's a flat molecule, and so these will 585 00:35:35,770 --> 00:35:37,610 stack on top of each other. 586 00:35:37,610 --> 00:35:39,770 So if they're stacked on top of each other 587 00:35:39,770 --> 00:35:42,010 and this is looking at them head on, 588 00:35:42,010 --> 00:35:45,340 you have this special pair of chlorophylls. 589 00:35:45,340 --> 00:35:48,910 You'll eventually-- you'll absorb a photon of light, 590 00:35:48,910 --> 00:35:55,610 and that will create a charge separation 591 00:35:55,610 --> 00:36:00,560 a cross between that special pair of chlorophylls. 592 00:36:00,560 --> 00:36:05,060 That electron, before it can decay as fluorescence, 593 00:36:05,060 --> 00:36:10,400 as would normally happen, it's very rapidly transferred out 594 00:36:10,400 --> 00:36:16,370 to a pheophytin molecule. 595 00:36:16,370 --> 00:36:20,150 And then it's very rapidly transferred again 596 00:36:20,150 --> 00:36:24,230 to a quinone called QA. 597 00:36:24,230 --> 00:36:28,230 All right, so just show that again here on here. 598 00:36:28,230 --> 00:36:30,480 So here's our special pair. 599 00:36:30,480 --> 00:36:34,020 You absorb a photon of light, charge separation 600 00:36:34,020 --> 00:36:37,680 at the special pair, electron transfer to the pheophytin, 601 00:36:37,680 --> 00:36:39,630 and then down to this quinone. 602 00:36:39,630 --> 00:36:41,333 And that's a very fast reaction. 603 00:36:41,333 --> 00:36:42,750 And you can see that it's actually 604 00:36:42,750 --> 00:36:50,550 moving within space that's roughly the distance from one 605 00:36:50,550 --> 00:36:52,780 side of the membrane to another. 606 00:36:52,780 --> 00:36:57,730 So on an atomic level, quite a long distance. 607 00:36:57,730 --> 00:36:59,820 And so from this quinone, it then 608 00:36:59,820 --> 00:37:01,980 will be transferred again to a second quinone. 609 00:37:01,980 --> 00:37:03,370 That's a slower step. 610 00:37:03,370 --> 00:37:05,790 I want to remind you how electrons 611 00:37:05,790 --> 00:37:07,530 are transferred on quinones. 612 00:37:07,530 --> 00:37:10,500 And so we talked about this already 613 00:37:10,500 --> 00:37:13,080 when we talked about electron transport. 614 00:37:13,080 --> 00:37:14,470 So here is a-- 615 00:37:17,310 --> 00:37:20,880 so this here would be an oxidized quinone 616 00:37:20,880 --> 00:37:25,350 just like we saw for coenzyme Q, ubiquinone. 617 00:37:25,350 --> 00:37:29,850 So this would be oxidized quinone and photosynthesis. 618 00:37:29,850 --> 00:37:44,810 Electron, you can pick up electrons one at a time, 619 00:37:44,810 --> 00:37:55,040 so that will generate this semiquinone. 620 00:37:55,040 --> 00:37:57,710 Remember, with the stabilized free radical, 621 00:37:57,710 --> 00:38:02,120 now you can pick up a second electron. 622 00:38:02,120 --> 00:38:08,510 And that ultimately generates the reduced, the fully reduced 623 00:38:08,510 --> 00:38:11,100 quinone, the quinol. 624 00:38:15,470 --> 00:38:17,480 We talked about this with coenzyme Q 625 00:38:17,480 --> 00:38:19,530 and oxidative phosphorylation. 626 00:38:19,530 --> 00:38:23,510 Remember, ubiquinone to ubiquinol, 627 00:38:23,510 --> 00:38:26,360 similar thing happening in photosynthesis. 628 00:38:26,360 --> 00:38:30,080 Also, a quinone can pick up electrons, go from the oxidized 629 00:38:30,080 --> 00:38:31,430 to the reduced state. 630 00:38:31,430 --> 00:38:35,360 And if you remember, in oxidative phosphorylation, 631 00:38:35,360 --> 00:38:37,760 you could then, as you transfer the electrons 632 00:38:37,760 --> 00:38:41,930 across from the oxidized and reduced state of the quinones, 633 00:38:41,930 --> 00:38:43,790 you'll pick up a proton. 634 00:38:43,790 --> 00:38:46,607 If you pick up those protons and release those protons 635 00:38:46,607 --> 00:38:48,440 as you transfer electrons on different sides 636 00:38:48,440 --> 00:38:50,330 of the membrane, that can be coupled 637 00:38:50,330 --> 00:38:53,210 to pump protons across the membrane 638 00:38:53,210 --> 00:38:57,920 as you transfer electrons across the system, exactly what 639 00:38:57,920 --> 00:39:00,230 happens here in photosynthesis. 640 00:39:00,230 --> 00:39:02,300 Except now that electron is basically 641 00:39:02,300 --> 00:39:04,867 coming from transfer of an excited 642 00:39:04,867 --> 00:39:06,950 electron from the special pair to the pheophytins, 643 00:39:06,950 --> 00:39:08,960 and ultimately to the quinones. 644 00:39:08,960 --> 00:39:12,350 And then from the quinones, can now be transferred down 645 00:39:12,350 --> 00:39:15,930 on electron transport chain. 646 00:39:15,930 --> 00:39:21,430 To show this in a quantum way, if you will-- 647 00:39:21,430 --> 00:39:33,870 so if this here is the special pair of chlorophyll 648 00:39:33,870 --> 00:39:38,690 and we draw here the pair of electrons 649 00:39:38,690 --> 00:39:42,710 in the special pair in the ground state, 650 00:39:42,710 --> 00:39:47,810 if you get a photon of light of the right energy, 651 00:39:47,810 --> 00:39:52,160 you excite one of those electrons 652 00:39:52,160 --> 00:39:55,460 here into a higher orbital. 653 00:39:55,460 --> 00:40:02,570 Before that can decay, if we transfer that electron out, 654 00:40:02,570 --> 00:40:06,320 basically take that electron and transfer it out, 655 00:40:06,320 --> 00:40:09,740 and instead here give it to the pheophytin-- 656 00:40:14,810 --> 00:40:19,970 so here's some orbital in the pheophytin-- it comes, 657 00:40:19,970 --> 00:40:23,960 it picks up this electron that's transferred away 658 00:40:23,960 --> 00:40:27,990 from the special pair. 659 00:40:27,990 --> 00:40:31,320 Well, now, what you have is you basically 660 00:40:31,320 --> 00:40:42,330 have a negative charge here and a positive charge 661 00:40:42,330 --> 00:40:43,590 up on the special pair. 662 00:40:43,590 --> 00:40:47,340 So you've created this charge separation. 663 00:40:47,340 --> 00:40:50,670 That electron can then be further transferred down 664 00:40:50,670 --> 00:40:57,320 the electron transport chain to the quinones, that 665 00:40:57,320 --> 00:41:00,790 then transfer them across the quinones, who 666 00:41:00,790 --> 00:41:04,060 are pumping proton. 667 00:41:04,060 --> 00:41:08,770 And ultimately, down an electron transport chain, 668 00:41:08,770 --> 00:41:11,980 ultimately with the final electron acceptor being 669 00:41:11,980 --> 00:41:14,530 in NADP+ to make NADPH. 670 00:41:19,950 --> 00:41:24,830 And you're still left with this positive charge 671 00:41:24,830 --> 00:41:26,840 on the special pair. 672 00:41:26,840 --> 00:41:30,030 Obviously, that's not a very stable state, 673 00:41:30,030 --> 00:41:32,240 so we've got to resolve that. 674 00:41:32,240 --> 00:41:34,110 How can that be resolved? 675 00:41:34,110 --> 00:41:37,640 Well, lots of things will-- that's a pretty good electron 676 00:41:37,640 --> 00:41:38,420 acceptor. 677 00:41:38,420 --> 00:41:43,740 And so now, even something like water, 678 00:41:43,740 --> 00:41:48,440 which is highly abundant, can be an electron donor, 679 00:41:48,440 --> 00:41:54,110 and ultimately fixes that positive charge 680 00:41:54,110 --> 00:41:58,520 on the special pair by taking electron from water, which, 681 00:41:58,520 --> 00:42:01,580 of course, generates oxygen. 682 00:42:01,580 --> 00:42:07,580 This part of the reaction is carried out by a complex called 683 00:42:07,580 --> 00:42:15,290 the water splitting complex- We: don't have time to get 684 00:42:15,290 --> 00:42:17,000 into this-- 685 00:42:17,000 --> 00:42:20,450 and some manganese-containing enzyme. 686 00:42:20,450 --> 00:42:24,890 And it basically controls the transfer 687 00:42:24,890 --> 00:42:29,660 of an electron from water to oxygen, which, hopefully, 688 00:42:29,660 --> 00:42:33,440 is at least intuitively you'll see why that's favorable, 689 00:42:33,440 --> 00:42:36,620 because you really have this true positive charge 690 00:42:36,620 --> 00:42:37,550 on the special pair. 691 00:42:37,550 --> 00:42:41,210 And almost anything is a good electron donor 692 00:42:41,210 --> 00:42:44,720 to that, including water. 693 00:42:44,720 --> 00:42:50,180 And so it's really this charge separation by fast electron 694 00:42:50,180 --> 00:42:52,820 transfer through the reaction center 695 00:42:52,820 --> 00:42:55,100 from the special chlorophyll pair 696 00:42:55,100 --> 00:43:00,380 ultimately down to quinones that creates this charged separation 697 00:43:00,380 --> 00:43:03,830 that is ultimately where you can pull an electron from water 698 00:43:03,830 --> 00:43:07,700 to generate oxygen. And if we come back over here, 699 00:43:07,700 --> 00:43:13,250 it's how we made this whole process work, how we made water 700 00:43:13,250 --> 00:43:14,810 on a good electron donor. 701 00:43:14,810 --> 00:43:16,700 That is, we didn't make the standard-- didn't 702 00:43:16,700 --> 00:43:18,740 change the standard reduction potential 703 00:43:18,740 --> 00:43:21,410 of the oxygen-water pair. 704 00:43:21,410 --> 00:43:24,680 Instead, what we did is we basically 705 00:43:24,680 --> 00:43:30,320 used light to excite an electron in chlorophyll such 706 00:43:30,320 --> 00:43:33,080 that that excited electron now can 707 00:43:33,080 --> 00:43:36,110 be favorably-- sits at a low enough standard reduction 708 00:43:36,110 --> 00:43:38,810 potential such that it's favorable to transfer 709 00:43:38,810 --> 00:43:40,400 it to NADPH. 710 00:43:40,400 --> 00:43:45,870 And then we fix that charge hole, if you will, 711 00:43:45,870 --> 00:43:48,410 by pulling an electron from water, 712 00:43:48,410 --> 00:43:52,970 and that generates oxygen. 713 00:43:52,970 --> 00:43:56,600 Of course, once we're able to get this favorable electron 714 00:43:56,600 --> 00:44:02,090 transfer, now we can couple that to the pumping of protons. 715 00:44:02,090 --> 00:44:04,340 That allows us to make delta psi, delta pH. 716 00:44:04,340 --> 00:44:07,850 We charge a battery, and that does work, 717 00:44:07,850 --> 00:44:11,570 just like we saw before. 718 00:44:11,570 --> 00:44:16,520 So why these concepts were so important to understanding 719 00:44:16,520 --> 00:44:19,760 how energy transduction and biological systems work, 720 00:44:19,760 --> 00:44:22,070 and why I spent so much time talking 721 00:44:22,070 --> 00:44:24,200 about standard reduction potentials, 722 00:44:24,200 --> 00:44:25,900 delta psi, delta pH, et cetera. 723 00:44:29,450 --> 00:44:33,770 Now, I want to describe these concepts 724 00:44:33,770 --> 00:44:38,390 and build them into the context of a chloroplast electron 725 00:44:38,390 --> 00:44:39,800 transport chain. 726 00:44:39,800 --> 00:44:43,760 I'm going to show you what happens in chloroplasts 727 00:44:43,760 --> 00:44:46,590 in higher plants. 728 00:44:46,590 --> 00:44:48,600 Conceptually, even though the details may not 729 00:44:48,600 --> 00:44:51,870 be 100% the same across all photosynthetic organisms, 730 00:44:51,870 --> 00:44:53,910 conceptually what I'm going to show you 731 00:44:53,910 --> 00:44:56,640 is exactly the same, regardless of 732 00:44:56,640 --> 00:44:58,830 whether it's a photosynthetic bacteria 733 00:44:58,830 --> 00:45:03,210 or a chloroplast from a plant. 734 00:45:03,210 --> 00:45:04,770 So if we draw here-- 735 00:45:04,770 --> 00:45:08,220 this here being the thylakoid membrane-- 736 00:45:17,190 --> 00:45:21,450 so I'm going to draw it such that this here is the stroma 737 00:45:21,450 --> 00:45:23,320 side of the membrane. 738 00:45:23,320 --> 00:45:27,120 This is the lumen side of the membrane. 739 00:45:27,120 --> 00:45:29,100 Remember, the stroma side would be 740 00:45:29,100 --> 00:45:34,560 equivalent to the matrix of the mitochondria, where the lumen 741 00:45:34,560 --> 00:45:38,940 side would be equivalent to the intermembrane space 742 00:45:38,940 --> 00:45:41,200 of the mitochondria. 743 00:45:41,200 --> 00:45:44,890 So thylakoid membrane, stroma side, lumen side. 744 00:45:48,490 --> 00:45:51,670 We have a complex here. 745 00:45:51,670 --> 00:46:07,340 This complex is called photosystem II, 746 00:46:07,340 --> 00:46:10,190 abbreviate PSII. 747 00:46:10,190 --> 00:46:15,470 Photosystem II is going to be linked up 748 00:46:15,470 --> 00:46:21,470 to the water splitting complex, which will basically pick up 749 00:46:21,470 --> 00:46:29,920 an electron, of course, injecting light that 750 00:46:29,920 --> 00:46:38,090 will transfer electrons through effectively 751 00:46:38,090 --> 00:46:42,590 what is a Q cycle, what we alluded to happens 752 00:46:42,590 --> 00:46:44,120 in oxidative phosphorylation-- 753 00:46:44,120 --> 00:46:47,030 I didn't have time to describe it in great detail-- 754 00:46:47,030 --> 00:46:52,890 that can pump protons across the membrane, 755 00:46:52,890 --> 00:46:58,080 as electrons are transferred into the next complex, which 756 00:46:58,080 --> 00:47:03,030 is called cytochrome bf. 757 00:47:03,030 --> 00:47:09,120 The next complex, cytochrome bf, also 758 00:47:09,120 --> 00:47:13,180 can pump protons across the membrane. 759 00:47:13,180 --> 00:47:17,920 Those electrons then get transferred 760 00:47:17,920 --> 00:47:24,640 to a soluble protein in the lumen called plastocyanin. 761 00:47:31,750 --> 00:47:34,930 From there, those electrons now get 762 00:47:34,930 --> 00:47:43,270 transferred to another complex called photosystem I, or PSI. 763 00:47:50,910 --> 00:48:00,960 Electrons in photosystem I can end up in another complex 764 00:48:00,960 --> 00:48:12,210 called ferredoxin, ultimately transferring those electrons 765 00:48:12,210 --> 00:48:17,730 to NADP+ to generate NADPH. 766 00:48:17,730 --> 00:48:19,980 And of course, that will happen here 767 00:48:19,980 --> 00:48:24,150 on the stromal side of the membrane, 768 00:48:24,150 --> 00:48:26,250 because that's where the dark reactions are 769 00:48:26,250 --> 00:48:27,630 going to take place. 770 00:48:27,630 --> 00:48:34,070 This has the effective generating a delta psi, 771 00:48:34,070 --> 00:48:40,640 delta pH across the membrane. 772 00:48:40,640 --> 00:48:44,270 And of course, that can be used then, 773 00:48:44,270 --> 00:48:56,110 that delta psi, delta pH can be used by a chloroplast F0, 774 00:48:56,110 --> 00:49:03,690 chloroplast F1 to generate ATP, just 775 00:49:03,690 --> 00:49:11,710 like we described from oxidative phosphorylation, 776 00:49:11,710 --> 00:49:16,180 with the ATP also being generated on the stromal side 777 00:49:16,180 --> 00:49:18,110 of the membrane. 778 00:49:18,110 --> 00:49:20,440 So this should look very similar to what 779 00:49:20,440 --> 00:49:24,860 we described for mitochondrial electron transport. 780 00:49:24,860 --> 00:49:29,800 So you see we have multiple complexes, including 781 00:49:29,800 --> 00:49:34,490 a cytochrome C-like protein that sits on, in this case, 782 00:49:34,490 --> 00:49:38,110 the lumen side of the thylakoid membrane, plastocyanin. 783 00:49:38,110 --> 00:49:40,360 Cytochrome C sits in the intermembrane space 784 00:49:40,360 --> 00:49:41,920 of the mitochondria. 785 00:49:41,920 --> 00:49:46,370 You transfer electrons, including via Q, 786 00:49:46,370 --> 00:49:48,970 quinone-like, cycle that pumps protons, 787 00:49:48,970 --> 00:49:51,760 have other complexes that pump protons, 788 00:49:51,760 --> 00:49:57,640 ultimately transfer electrons to generate NADPH. 789 00:49:57,640 --> 00:50:04,240 There's two complexes here that absorb light, photosystem II 790 00:50:04,240 --> 00:50:09,440 and photosystem I. Electron transfer, I guess, classically, 791 00:50:09,440 --> 00:50:13,430 would go photosystem II, cytochrome bf, plastocyanin, 792 00:50:13,430 --> 00:50:16,310 photosystem I, ferredoxin. 793 00:50:16,310 --> 00:50:20,510 Water is the donor, NADPH as the acceptor, 794 00:50:20,510 --> 00:50:22,580 generates delta psi, delta pH. 795 00:50:22,580 --> 00:50:24,800 That delta psi, delta pH can then 796 00:50:24,800 --> 00:50:29,450 be used to do work, including synthesis of ATP, as done 797 00:50:29,450 --> 00:50:35,020 by an F0F1-ATPase, just like exists in the mitochondria. 798 00:50:35,020 --> 00:50:36,910 Of course, this evolved first. 799 00:50:36,910 --> 00:50:41,410 And so oxidative phosphorylation looks like photosynthesis, 800 00:50:41,410 --> 00:50:44,260 even though we talked about it, oxidative phosphorylation 801 00:50:44,260 --> 00:50:46,070 first and then photosynthesis. 802 00:50:46,070 --> 00:50:50,110 So you will, of course, looks like oxidative phosphorylation, 803 00:50:50,110 --> 00:50:52,570 but, of course, remember, it's really 804 00:50:52,570 --> 00:50:55,420 not that photosynthesis looks like oxidative phosphorylation, 805 00:50:55,420 --> 00:50:57,130 it's that oxidative phosphorylation looks 806 00:50:57,130 --> 00:51:00,070 like photosynthesis. 807 00:51:00,070 --> 00:51:04,210 Now ultimately, the reason this works is because photosystem II 808 00:51:04,210 --> 00:51:10,690 and photosystem I can absorb photons, which ultimately, 809 00:51:10,690 --> 00:51:13,660 by the processes we discussed, is what makes electron transfer 810 00:51:13,660 --> 00:51:20,085 favorable from water to NADP+ to generate oxygen and NADPH. 811 00:51:22,840 --> 00:51:25,240 Now, I want to take you through just 812 00:51:25,240 --> 00:51:28,120 to show you how this works, because it turns out 813 00:51:28,120 --> 00:51:30,640 it's also possible to short circuit 814 00:51:30,640 --> 00:51:34,210 this in a way that can favor more delta psi, 815 00:51:34,210 --> 00:51:38,030 delta pH production relative to NADPH production, 816 00:51:38,030 --> 00:51:40,390 which is important, because you can imagine 817 00:51:40,390 --> 00:51:44,200 that a photosynthetic organism has to balance 818 00:51:44,200 --> 00:51:48,850 its needs for NADPH and ATP-- 819 00:51:48,850 --> 00:51:51,130 both useful molecules, but you want 820 00:51:51,130 --> 00:51:53,270 to balance your needs for both of them. 821 00:51:53,270 --> 00:51:57,700 And it turns out that this middle part, photosystem I, 822 00:51:57,700 --> 00:52:01,480 works with cytochrome bf in a way that 823 00:52:01,480 --> 00:52:03,790 allows it to balance that. 824 00:52:46,910 --> 00:52:53,040 So if I draw this out in a little bit more detail-- 825 00:52:53,040 --> 00:53:00,545 so here's negative 1.6 volts and here's positive 0.8 volts. 826 00:53:12,130 --> 00:53:16,940 So that's, again, our standard reduction potential. 827 00:53:16,940 --> 00:53:23,830 So remember, water and oxygen is going to sit down here. 828 00:53:23,830 --> 00:53:27,120 You're going to have in photosystem I 829 00:53:27,120 --> 00:53:30,240 a special pair called P680. 830 00:53:30,240 --> 00:53:33,630 That's the wavelength of light. 831 00:53:38,960 --> 00:53:43,100 At the special pair in-- 832 00:53:43,100 --> 00:53:45,695 so this here would be photosystem-- 833 00:53:45,695 --> 00:53:47,480 sorry, photosystem II. 834 00:53:47,480 --> 00:53:54,680 That's a special pair within photosystem II, absorbs photon. 835 00:53:54,680 --> 00:54:02,030 That photon now becomes an excited electron 836 00:54:02,030 --> 00:54:04,280 that can be transferred away. 837 00:54:04,280 --> 00:54:08,570 That ultimately makes water a good electron donor 838 00:54:08,570 --> 00:54:11,060 to resolve that charge separation 839 00:54:11,060 --> 00:54:12,800 at this special pair. 840 00:54:12,800 --> 00:54:16,760 That electron is ultimately transferred away, coming down 841 00:54:16,760 --> 00:54:20,390 here to cytochrome bf. 842 00:54:20,390 --> 00:54:26,640 So this moves from a higher to a lower standard reduction 843 00:54:26,640 --> 00:54:27,940 potential that's favorable. 844 00:54:27,940 --> 00:54:35,820 So this should go here through the pheophytin, QA, QB, et 845 00:54:35,820 --> 00:54:39,690 cetera, through a Q-like cycle to get 846 00:54:39,690 --> 00:54:43,260 to cytochrome bf and pump a proton. 847 00:54:43,260 --> 00:54:48,600 That would then transfer to plastocyanin. 848 00:54:48,600 --> 00:54:56,660 That would then transfer, ultimately, to photosystem I. 849 00:54:56,660 --> 00:55:07,310 So there's photosystem I, which also can absorb a photon, 850 00:55:07,310 --> 00:55:16,400 excite that electron, which can be rapidly transferred away. 851 00:55:16,400 --> 00:55:19,670 But this time, rather than water being the electron donor, 852 00:55:19,670 --> 00:55:23,750 it's the electron transfer from plastocyanin that resolves 853 00:55:23,750 --> 00:55:27,980 the charged separation on the special pair as you excite 854 00:55:27,980 --> 00:55:33,470 and transfer the electrons out, ultimately coming here 855 00:55:33,470 --> 00:55:43,460 to ferredoxin, which is at the right standard reduction 856 00:55:43,460 --> 00:55:51,820 potential to reduce NADP+ to NADPH. 857 00:55:51,820 --> 00:55:58,030 And so this here also would come through quinones. 858 00:55:58,030 --> 00:56:01,590 A0, A1 is what those quinones are called in photosystem 859 00:56:01,590 --> 00:56:02,590 I. That's not important. 860 00:56:02,590 --> 00:56:05,560 It's exactly the same thing as what we described before. 861 00:56:05,560 --> 00:56:09,640 And ultimately, this is drawing out, basically, 862 00:56:09,640 --> 00:56:11,950 what happens in the chloroplasts, how photosystem 863 00:56:11,950 --> 00:56:15,940 II and photosystem I fit in with electron transfer in terms 864 00:56:15,940 --> 00:56:19,220 of moving across standard reduction potentials. 865 00:56:19,220 --> 00:56:23,710 Now, what's cool is that photosystem I, 866 00:56:23,710 --> 00:56:28,600 rather than transferring the electrons to make NADPH, 867 00:56:28,600 --> 00:56:34,750 also can instead transfer electrons as an alternative 868 00:56:34,750 --> 00:56:38,170 back to cytochrome bf. 869 00:56:38,170 --> 00:56:41,050 Well, if it comes back into cytochrome bf, 870 00:56:41,050 --> 00:56:43,360 you can see that basically this now 871 00:56:43,360 --> 00:56:47,560 creates a loop where you have favorable electron transfer 872 00:56:47,560 --> 00:56:50,920 here, excite, favorable electron transfer, excite, 873 00:56:50,920 --> 00:56:52,990 favorable electron transfer. 874 00:56:52,990 --> 00:56:58,690 In essence, this here will then pump a proton 875 00:56:58,690 --> 00:57:02,530 by basically transferring the electron back, 876 00:57:02,530 --> 00:57:07,420 allowing you to run this middle part between photosystem 877 00:57:07,420 --> 00:57:11,950 I, cytochrome bf, and plastocyanin, create delta psi, 878 00:57:11,950 --> 00:57:14,840 delta pH at cytochrome bf. 879 00:57:14,840 --> 00:57:19,010 And that allows you to make ATP without generating NADPH, 880 00:57:19,010 --> 00:57:21,610 or make a battery, delta psi, delta pH, 881 00:57:21,610 --> 00:57:26,860 do whatever work without generating NADPH. 882 00:57:26,860 --> 00:57:31,270 That's also shown here on this slide, drawn out 883 00:57:31,270 --> 00:57:34,420 in a way that may be slightly neater than what 884 00:57:34,420 --> 00:57:37,960 I drew out over there, but effectively illustrates 885 00:57:37,960 --> 00:57:38,680 the same thing. 886 00:57:38,680 --> 00:57:42,280 And it's cool that there's this flexibility 887 00:57:42,280 --> 00:57:45,790 to run photosynthesis in a way that you can net generate 888 00:57:45,790 --> 00:57:51,070 NADPH, but you can also generate just delta psi, delta pH, 889 00:57:51,070 --> 00:57:54,520 allowing the plant to meet its energetic needs, 890 00:57:54,520 --> 00:57:58,510 because delta psi, delta pH can keep ATP-ADP ratio high 891 00:57:58,510 --> 00:58:00,070 when the lights are on. 892 00:58:00,070 --> 00:58:02,590 And then that way, the plant has all the energy 893 00:58:02,590 --> 00:58:08,060 it needs to fight entropy and be alive as a cell. 894 00:58:08,060 --> 00:58:11,450 Whereas, it can also tune it, then, to generate the NADPH 895 00:58:11,450 --> 00:58:14,090 it needs to make reduced carbon that it can then 896 00:58:14,090 --> 00:58:17,530 store for later. 897 00:58:17,530 --> 00:58:21,820 Now, to make the reduced carbon that can be stored for later, 898 00:58:21,820 --> 00:58:27,100 ultimately, now we have to basically take CO2 and turn it 899 00:58:27,100 --> 00:58:29,725 into carbohydrate, turn it into glucose. 900 00:58:35,380 --> 00:58:41,410 This CO2 into carbohydrate, turning it into glucose, 901 00:58:41,410 --> 00:58:44,560 that hexo-sugar that the plant can then pack away with 902 00:58:44,560 --> 00:58:50,140 alpha-1,4 linkages as starch to burn later when the lights are 903 00:58:50,140 --> 00:58:53,260 off, requires reduction. 904 00:58:53,260 --> 00:58:55,930 That reduction electrons have to come from somewhere. 905 00:58:55,930 --> 00:59:01,280 Well, ultimately, they can come from water via NADPH. 906 00:59:01,280 --> 00:59:05,660 And those are the dark reactions of photosynthesis. 907 00:59:05,660 --> 00:59:08,020 And so let's discuss those next. 908 00:59:08,020 --> 00:59:10,300 And so the dark reactions, really, 909 00:59:10,300 --> 00:59:28,760 are CO2 plus NADPH going to carbohydrate plus NADP+. 910 00:59:32,240 --> 00:59:35,750 These are reactions that can happen either 911 00:59:35,750 --> 00:59:37,700 in the dark or the light. 912 00:59:37,700 --> 00:59:41,270 They're unique to photosynthetic organisms. 913 00:59:41,270 --> 00:59:47,750 That's who can net carry out this reaction, 914 00:59:47,750 --> 00:59:52,190 fixing carbon as glucose, as reduced carbon and glucose. 915 00:59:52,190 --> 00:59:54,890 These reactions, these dark reactions, 916 00:59:54,890 --> 01:00:01,370 are going to occur here in the stroma of the chloroplast. 917 01:00:01,370 --> 01:00:02,930 And why is that useful? 918 01:00:02,930 --> 01:00:06,440 Because this is where the NADPH and the ATP that's 919 01:00:06,440 --> 01:00:10,370 going to be required for these dark reactions to take place. 920 01:00:10,370 --> 01:00:13,280 It should be clear to you, NADPH is the electron donor, 921 01:00:13,280 --> 01:00:15,770 but you also need additional energy input 922 01:00:15,770 --> 01:00:19,370 because this is effectively reversing glucose oxidation. 923 01:00:19,370 --> 01:00:21,632 That releases energy, need energy input, 924 01:00:21,632 --> 01:00:23,090 and so you're going to see you also 925 01:00:23,090 --> 01:00:27,920 need ATP to ADP conversion. 926 01:00:27,920 --> 01:00:31,190 By happening in the stroma, you have the place 927 01:00:31,190 --> 01:00:34,430 where the NADPH and ATP is being produced as products 928 01:00:34,430 --> 01:00:37,180 of the light reaction. 929 01:00:37,180 --> 01:00:40,120 So these dark reactions of photosynthesis 930 01:00:40,120 --> 01:00:44,200 were discovered in the 1950s by a gentleman 931 01:00:44,200 --> 01:00:46,630 by the name of Melvin Calvin. 932 01:00:46,630 --> 01:00:50,830 As a result, sometimes the dark reactions 933 01:00:50,830 --> 01:00:55,390 are referred to as the Calvin cycle 934 01:00:55,390 --> 01:00:58,030 to honor him as the discoverer. 935 01:00:58,030 --> 01:01:02,980 And basically, the experiment that Calvin did is he took 936 01:01:02,980 --> 01:01:06,830 carbon dioxide that was labeled with radioactive carbon-- 937 01:01:06,830 --> 01:01:09,670 so C14 carbon dioxide-- 938 01:01:09,670 --> 01:01:14,350 he basically fed it to a photosynthetic algae 939 01:01:14,350 --> 01:01:17,530 and found that the first compound that 940 01:01:17,530 --> 01:01:23,180 incorporated the radioactive CO2 was a 3-carbon compound. 941 01:01:23,180 --> 01:01:26,470 And that 3-carbon compound was 3-phosphogylcerate. 942 01:01:32,110 --> 01:01:35,480 Good old 3-PG from glycolysis. 943 01:01:35,480 --> 01:01:40,100 And so he found that you could get these reactions of CO2 944 01:01:40,100 --> 01:01:43,160 going into 3-phosphoglycerate is the first thing 945 01:01:43,160 --> 01:01:45,500 to incorporate the radioactive carbon. 946 01:01:45,500 --> 01:01:47,450 That worked in the light or the dark, 947 01:01:47,450 --> 01:01:51,900 and that's where the term dark reactions came from. 948 01:01:51,900 --> 01:01:54,500 Now, it should be, hopefully, clear to you 949 01:01:54,500 --> 01:01:58,860 that if I can generate 3-phosphoglycerate, 950 01:01:58,860 --> 01:02:00,570 you now know how to make glucose. 951 01:02:00,570 --> 01:02:02,653 Because now you can go back and look at your notes 952 01:02:02,653 --> 01:02:04,350 from the gluconeogenesis lecture, 953 01:02:04,350 --> 01:02:07,020 and if you had a source of 3-phosphoglycerate, 954 01:02:07,020 --> 01:02:11,430 you can just run it through the reactions of gluconeogenesis, 955 01:02:11,430 --> 01:02:15,850 and ultimately make glucose. 956 01:02:15,850 --> 01:02:21,070 Turns out that the way this happens in total 957 01:02:21,070 --> 01:02:27,670 is the so-called Calvin cycle, really happens in three phases. 958 01:02:27,670 --> 01:02:32,090 So the first phase is called fixation. 959 01:02:32,090 --> 01:02:34,420 And so what is fixation? 960 01:02:34,420 --> 01:02:36,550 That's basically Calvin's experiment. 961 01:02:36,550 --> 01:02:44,360 It's using CO2 to generate 3-phosphoglycerate. 962 01:02:44,360 --> 01:02:49,940 The second phase is referred to as reduction. 963 01:02:49,940 --> 01:02:51,020 So what's reduction? 964 01:02:51,020 --> 01:02:53,930 Well, that's basically gluconeogenesis. 965 01:02:53,930 --> 01:03:00,740 So it's using 3-phosphoglycerate and turning it into glucose. 966 01:03:00,740 --> 01:03:03,097 Gluconeogenesis, we're reducing carbon. 967 01:03:03,097 --> 01:03:05,180 This is the step, of course, so you need electrons 968 01:03:05,180 --> 01:03:05,805 from somewhere. 969 01:03:05,805 --> 01:03:07,347 They're going to come from NADPH. 970 01:03:07,347 --> 01:03:08,870 We need NADPH for that. 971 01:03:08,870 --> 01:03:11,990 You'll also need ATP. 972 01:03:11,990 --> 01:03:17,750 And then the last step is called regeneration. 973 01:03:17,750 --> 01:03:22,190 And what you will see is that the Calvin cycle is a cycle. 974 01:03:22,190 --> 01:03:25,610 That is, you're going to have to have an entry point and an exit 975 01:03:25,610 --> 01:03:26,270 point. 976 01:03:26,270 --> 01:03:28,790 And just like we talked about for the TCA cycle, 977 01:03:28,790 --> 01:03:32,510 if we feed two carbon acetyl CoA units into the TCA cycle, 978 01:03:32,510 --> 01:03:35,840 we have to have an acceptor, oxaloacetate, in order 979 01:03:35,840 --> 01:03:37,260 to run the cycle. 980 01:03:37,260 --> 01:03:39,110 And if we pull anything out of the cycle, 981 01:03:39,110 --> 01:03:42,090 we need another way to add carbon back to the cycle. 982 01:03:42,090 --> 01:03:48,500 And so regeneration is really anaplerosis for the cycle. 983 01:03:48,500 --> 01:03:51,260 That is, you need to generate the acceptor that you can 984 01:03:51,260 --> 01:03:54,170 keep running this as a cycle. 985 01:03:54,170 --> 01:03:56,700 As a general overview, it works as follows. 986 01:03:56,700 --> 01:04:07,160 And so it turns out that the cycle works with this molecule 987 01:04:07,160 --> 01:04:12,260 as, I guess, if we're going to draw an analogy to the TCA 988 01:04:12,260 --> 01:04:17,840 cycle, where oxaloacetate was the initial acceptor, 989 01:04:17,840 --> 01:04:20,930 this is the analogous molecule to that. 990 01:04:20,930 --> 01:04:23,450 This is a molecule, it's a 5-carbon sugar. 991 01:04:23,450 --> 01:04:26,180 So it's pentose phosphorylated-- it's 992 01:04:26,180 --> 01:04:29,450 a pentose and a ketose phosphorylated 993 01:04:29,450 --> 01:04:32,010 on the one and the five position. 994 01:04:32,010 --> 01:04:40,140 So this is ribulose 1,5-bisphosphate, 995 01:04:40,140 --> 01:04:44,100 which I will abbreviate R15P. 996 01:04:46,980 --> 01:04:53,640 So ribulose 1,5-bisphosphate will pick up a CO2 molecule, 997 01:04:53,640 --> 01:05:07,530 and it will make two 3-phosphoglycerate molecules. 998 01:05:07,530 --> 01:05:10,560 So five carbons plus one carbon equals six carbons. 999 01:05:10,560 --> 01:05:14,640 Break them in half, two 3-phosphoglycerate molecules. 1000 01:05:14,640 --> 01:05:17,280 Once you have that 3-phosphoglycerate, 1001 01:05:17,280 --> 01:05:25,740 I can now use ATP to phosphorylate 1002 01:05:25,740 --> 01:05:27,660 that 3-phosphoglycerate. 1003 01:05:27,660 --> 01:05:30,780 That'll give me 1,3-bisphosphoglycerate. 1004 01:05:34,390 --> 01:05:41,440 I can now run the photosynthetic version of the GAPDH reaction, 1005 01:05:41,440 --> 01:05:43,390 which is exactly the reaction that you 1006 01:05:43,390 --> 01:05:45,100 saw in glycolysis, except now we're 1007 01:05:45,100 --> 01:05:49,420 going to use NADPH rather than NADH, 1008 01:05:49,420 --> 01:05:54,660 as we described for glycolysis and gluconeogenesis. 1009 01:05:54,660 --> 01:05:58,790 And so that's going to give me glyceraldehyde 1010 01:05:58,790 --> 01:06:04,050 3-phosphate, glyceraldehyde 3-phosphate, 1011 01:06:04,050 --> 01:06:06,030 use triose-phosphate isomerase to make 1012 01:06:06,030 --> 01:06:08,770 dihydroxyacetone phosphate. 1013 01:06:08,770 --> 01:06:14,752 And I can run aldolase reaction to make fructose 1014 01:06:14,752 --> 01:06:19,325 1,6-bisphosphate, can release a phosphate from fructose 1015 01:06:19,325 --> 01:06:20,610 1,6-bisphosphate. 1016 01:06:20,610 --> 01:06:25,020 Now I have fructose 6-phosphate. 1017 01:06:25,020 --> 01:06:28,170 I can isomerize fructose 6-phosphate 1018 01:06:28,170 --> 01:06:30,660 to glucose 6-phosphate. 1019 01:06:30,660 --> 01:06:34,173 And of course, if I take the phosphate off, that's glucose. 1020 01:06:34,173 --> 01:06:36,090 Or, more importantly, what would the plant do? 1021 01:06:36,090 --> 01:06:38,670 It would then do a mutase reaction 1022 01:06:38,670 --> 01:06:42,960 to make glucose 1-phosphate and pack that away in starch, 1023 01:06:42,960 --> 01:06:46,740 much like we discussed for putting glucose into glycogen. 1024 01:06:46,740 --> 01:06:48,720 And then get that glucose back out 1025 01:06:48,720 --> 01:06:50,850 as glucose 1-phosphate, glucose 6-phosphate, 1026 01:06:50,850 --> 01:06:56,160 and send it to glycolysis to be oxidized to get energy. 1027 01:06:56,160 --> 01:06:59,790 This is great, but you can see that this is not going to work 1028 01:06:59,790 --> 01:07:05,600 unless I can regenerate a source of ribulose 1,5-bisphosphate. 1029 01:07:05,600 --> 01:07:09,270 Turns out, you can do that from fructose 6-phosphate. 1030 01:07:09,270 --> 01:07:12,840 Obviously, ATP is going to be required here, too, 1031 01:07:12,840 --> 01:07:14,970 because there's only one phosphate here. 1032 01:07:14,970 --> 01:07:20,100 You need two phosphates for the ribulose 1,5-bisphosphate. 1033 01:07:20,100 --> 01:07:21,810 So more ATP is needed. 1034 01:07:24,750 --> 01:07:30,720 And ultimately, that, in a non-stoichiometric way, 1035 01:07:30,720 --> 01:07:33,100 is how the cycle works. 1036 01:07:33,100 --> 01:07:46,000 And so if we break this up then into the different phases, 1037 01:07:46,000 --> 01:07:52,030 this phase up here would be fixation, 1038 01:07:52,030 --> 01:07:57,340 this phase down here is reduction, 1039 01:07:57,340 --> 01:08:01,500 and this phase over here is regeneration. 1040 01:08:04,590 --> 01:08:09,862 And that's basically the Calvin cycle. 1041 01:08:09,862 --> 01:08:11,570 Let's go through these now one at a time. 1042 01:08:11,570 --> 01:08:13,225 Let's first talk about fixation. 1043 01:08:18,700 --> 01:08:25,370 Fixation is catalyzed by an enzyme called RuBisCO, 1044 01:08:25,370 --> 01:08:32,850 which stands for ribulose-1,5-bisphosphate 1045 01:08:32,850 --> 01:08:34,824 carboxylase-oxygenase. 1046 01:08:42,470 --> 01:08:45,620 So RuBisCO, if you want a bit of trivia, 1047 01:08:45,620 --> 01:08:48,170 is the most abundant enzyme on earth. 1048 01:08:48,170 --> 01:08:52,250 It's about 50% of the protein in chloroplasts. 1049 01:08:52,250 --> 01:08:54,950 It's an incredibly inefficient enzyme. 1050 01:08:54,950 --> 01:08:57,750 It has a turnover number about three per second. 1051 01:08:57,750 --> 01:09:00,800 And that's why it's so abundant, because it's a crappy enzyme. 1052 01:09:00,800 --> 01:09:03,319 And so therefore, it needs to have a lot of it around 1053 01:09:03,319 --> 01:09:04,670 in order to work. 1054 01:09:04,670 --> 01:09:07,640 It evolved in a pre-oxygen atmosphere, 1055 01:09:07,640 --> 01:09:11,029 and now, of course, there's lots of oxygen in the atmosphere. 1056 01:09:11,029 --> 01:09:13,760 And CO2 and oxygen will both compete 1057 01:09:13,760 --> 01:09:15,420 to react with the enzyme. 1058 01:09:15,420 --> 01:09:18,680 And it turns out that that's a issue for photosynthesis, 1059 01:09:18,680 --> 01:09:22,720 and it's a big deal for agriculture. 1060 01:09:22,720 --> 01:09:25,630 Now, the way RuBisCO works is that there 1061 01:09:25,630 --> 01:09:31,840 is a lysine in the active site of the enzyme. 1062 01:09:31,840 --> 01:09:39,100 And that lysine is covalently bound to a CO2 molecule. 1063 01:09:39,100 --> 01:09:42,130 And that CO2 molecule covalently bound to the lysine 1064 01:09:42,130 --> 01:09:43,497 is not involved in the reaction. 1065 01:09:43,497 --> 01:09:45,580 But basically, if you don't have enough CO2 around 1066 01:09:45,580 --> 01:09:48,550 to carboxylate the lysine, now you 1067 01:09:48,550 --> 01:09:50,050 can't carry out the reaction. 1068 01:09:50,050 --> 01:09:52,689 It's one way to ensure that there's 1069 01:09:52,689 --> 01:09:54,730 enough CO2 that this will work. 1070 01:09:54,730 --> 01:09:58,660 But effectively, what that CO2 does is coordinates a magnesium 1071 01:09:58,660 --> 01:10:06,325 atom that ultimately coordinates and positions the ribulose 1072 01:10:06,325 --> 01:10:14,440 1,5-bisphosphate within the active site of the enzyme. 1073 01:10:23,640 --> 01:10:26,880 So here is ribulose 1,5-bisphosphate bound 1074 01:10:26,880 --> 01:10:31,840 into the active site of RuBisCO. 1075 01:10:31,840 --> 01:10:36,320 I'll show you quickly how the RuBisCO reaction works. 1076 01:10:36,320 --> 01:10:45,340 And so if we take the enol-- 1077 01:10:45,340 --> 01:10:53,060 or, the keto form of ribulose 1,5-bisphosphate and redraw it 1078 01:10:53,060 --> 01:11:13,240 in the active site as the keto form so it's-- 1079 01:11:20,130 --> 01:11:22,300 that will react with CO2. 1080 01:11:59,925 --> 01:12:00,790 You add water. 1081 01:12:45,180 --> 01:12:49,320 And then ultimately, what we are left with from the top half 1082 01:12:49,320 --> 01:13:04,160 of the molecule, we get this 1,3-phosphoglycerate molecule, 1083 01:13:04,160 --> 01:13:17,010 and on the bottom part also get a 3-phosphoglycerate molecule. 1084 01:13:17,010 --> 01:13:23,840 So two 3-phosphoglycerates generated. 1085 01:13:23,840 --> 01:13:29,330 So ultimately, what's happening is we're adding a CO2 basically 1086 01:13:29,330 --> 01:13:37,540 between the two and three carbon of ribulose 1,5-bisphophate, 1087 01:13:37,540 --> 01:13:41,650 breaking the molecule in half to get two 3-phosphoglycerate 1088 01:13:41,650 --> 01:13:43,460 molecules. 1089 01:13:43,460 --> 01:13:49,640 Just to quickly show you what happens if at this step 1090 01:13:49,640 --> 01:13:58,610 if you instead replace this with oxygen, so instead have oxygen 1091 01:13:58,610 --> 01:14:00,170 add there instead. 1092 01:14:00,170 --> 01:14:03,170 Well, now you end up with this situation. 1093 01:14:26,330 --> 01:14:28,480 So now you have that instead. 1094 01:14:28,480 --> 01:14:41,330 Now, when water gets added, you end up 1095 01:14:41,330 --> 01:14:48,040 generating from the bottom half of the molecule, 1096 01:14:48,040 --> 01:14:53,380 of course, a 3-phosphoglycerate. 1097 01:14:53,380 --> 01:14:56,940 So that's not a problem. 1098 01:14:56,940 --> 01:14:59,770 But from the top half of the molecule, 1099 01:14:59,770 --> 01:15:02,200 rather than generating a 3-phosphoglycerate, now 1100 01:15:02,200 --> 01:15:11,440 you have this 2-carbon unit, which 1101 01:15:11,440 --> 01:15:13,775 is called phosphoglycolate. 1102 01:15:19,650 --> 01:15:24,150 And it turns out that phosphoglycolate is not 1103 01:15:24,150 --> 01:15:26,760 a good thing for the plant to have. 1104 01:15:26,760 --> 01:15:30,565 You basically started with 5-carbon ribulose 1105 01:15:30,565 --> 01:15:35,550 1,5-bisphosphate, and you end up with 3-phosphoglycerate 1106 01:15:35,550 --> 01:15:39,300 and a 2-carbon phosphoglycolate. 1107 01:15:39,300 --> 01:15:42,840 No carbon is added there, and now the plant 1108 01:15:42,840 --> 01:15:45,810 has to deal with the phosphoglycolate. 1109 01:15:45,810 --> 01:15:49,230 So it has to regenerate the ribulose 1,5-bisphophate. 1110 01:15:49,230 --> 01:15:52,230 So naturally, if you're going to take-- 1111 01:15:52,230 --> 01:15:54,840 rebuild that 5-carbon molecule, that's 1112 01:15:54,840 --> 01:15:58,740 going to require energy input. 1113 01:15:58,740 --> 01:16:01,920 And you haven't fixed a CO2. 1114 01:16:01,920 --> 01:16:07,320 So in the end, what's required is actually more ATP and NADPH 1115 01:16:07,320 --> 01:16:11,370 to solve the phosphoglycolate problem 1116 01:16:11,370 --> 01:16:14,560 with no net gain for the plant. 1117 01:16:14,560 --> 01:16:20,190 And so as a result, oxygen competing with carbon dioxide 1118 01:16:20,190 --> 01:16:22,020 is a big deal for plants. 1119 01:16:22,020 --> 01:16:25,020 By the way, dealing with phosphoglycolate-- which, 1120 01:16:25,020 --> 01:16:26,940 of course, plants have a way to do-- 1121 01:16:26,940 --> 01:16:29,370 is a process called photorespiration. 1122 01:16:29,370 --> 01:16:33,720 Photorespiration requires all this ATP and NADPH, 1123 01:16:33,720 --> 01:16:35,670 so it requires all this extra light energy 1124 01:16:35,670 --> 01:16:37,800 from the plant that's really not being 1125 01:16:37,800 --> 01:16:43,900 used for any good purpose for the plant itself. 1126 01:16:43,900 --> 01:16:48,690 Now, the process I described is the standard photosynthesis 1127 01:16:48,690 --> 01:16:54,420 process, and it's what happens in so-called C3 plants, called 1128 01:16:54,420 --> 01:16:58,170 C3 because a 3-carbon intermediate is made. 1129 01:16:58,170 --> 01:17:01,800 It turns out that there is also a class of plants 1130 01:17:01,800 --> 01:17:07,200 called C4 plants that basically use a 4-carbon intermediate. 1131 01:17:07,200 --> 01:17:09,060 They do the same thing 3 plants do, 1132 01:17:09,060 --> 01:17:12,120 but they basically have a system that basically 1133 01:17:12,120 --> 01:17:16,920 uses some extra ATP and NADPH in order 1134 01:17:16,920 --> 01:17:22,050 to generate a shuttle to concentrate 1135 01:17:22,050 --> 01:17:26,400 CO2 to run the RuBisCO reaction that I just showed. 1136 01:17:26,400 --> 01:17:28,660 And so that is shown here on this slide. 1137 01:17:28,660 --> 01:17:31,350 And so this is what happens in C4 plants. 1138 01:17:31,350 --> 01:17:36,180 And so effectively, you fix a CO2 using-- 1139 01:17:36,180 --> 01:17:38,680 basically, making oxaloacetate. 1140 01:17:38,680 --> 01:17:40,350 So that's the 4-carbon unit. 1141 01:17:40,350 --> 01:17:44,670 You run an NADPH-driven version of the malate dehydrogenase 1142 01:17:44,670 --> 01:17:47,040 reaction to ultimately generate malate. 1143 01:17:47,040 --> 01:17:49,320 And then turn that malate back into pyruvate, 1144 01:17:49,320 --> 01:17:53,400 releasing CO2 back over here into the different parts. 1145 01:17:53,400 --> 01:17:55,980 So use a PEPCK-like reaction, send it 1146 01:17:55,980 --> 01:18:00,240 to the chloroplast, where you then regenerate 1147 01:18:00,240 --> 01:18:02,580 CO2 for the RuBisCO enzyme. 1148 01:18:02,580 --> 01:18:04,980 And you send that pyruvate back out as a way 1149 01:18:04,980 --> 01:18:09,450 to run a cycle that basically concentrates the CO2. 1150 01:18:09,450 --> 01:18:14,460 So this requires extra ATP, requires extra NADPH. 1151 01:18:14,460 --> 01:18:16,260 But in the end, it saves the plant 1152 01:18:16,260 --> 01:18:19,920 the trouble of having to deal with this phosphoglycolate. 1153 01:18:19,920 --> 01:18:21,780 Not surprisingly, this has evolved 1154 01:18:21,780 --> 01:18:23,100 in places with high light. 1155 01:18:23,100 --> 01:18:26,160 So tropical plants are more likely to be C4 plants 1156 01:18:26,160 --> 01:18:28,210 because they have more light around, more ATP, 1157 01:18:28,210 --> 01:18:33,300 NADPH any way, that allows them to compete with each other 1158 01:18:33,300 --> 01:18:36,360 and concentrate CO2 in a way that allows them 1159 01:18:36,360 --> 01:18:40,200 to run the RuBisCO reaction. 1160 01:18:40,200 --> 01:18:43,900 All right, reduction, the next phase up here. 1161 01:18:43,900 --> 01:18:46,020 There's actually not a lot to say. 1162 01:18:46,020 --> 01:18:52,230 This is basically just gluconeogenesis using NADPH. 1163 01:18:52,230 --> 01:18:55,080 And of course, you can use it to generate glucose, 1164 01:18:55,080 --> 01:18:58,410 but plants would rather generate glucose 1-phosphate 1165 01:18:58,410 --> 01:19:00,930 and store it as starch or something 1166 01:19:00,930 --> 01:19:05,470 else, some storage sugar for the plant itself. 1167 01:19:05,470 --> 01:19:11,200 Now, what should be clear to you is that, well, we 1168 01:19:11,200 --> 01:19:14,680 started with a 5-carbon unit and we added a 1-carbon unit. 1169 01:19:14,680 --> 01:19:23,390 And in the end, if we're going to net generate a glucose, 1170 01:19:23,390 --> 01:19:27,410 we're going to have to run that cycle six times, because we 1171 01:19:27,410 --> 01:19:30,170 need a 5-carbon unit to come out the end, 1172 01:19:30,170 --> 01:19:36,230 as well as build a 6-carbon unit just from CO2. 1173 01:19:36,230 --> 01:19:44,630 And so regeneration is really how do we go from net six CO2 1174 01:19:44,630 --> 01:19:47,570 to one glucose molecule. 1175 01:19:47,570 --> 01:19:52,910 That is, how can we combine that reduction step while also 1176 01:19:52,910 --> 01:19:57,950 regenerating ribulose 1,5-bisphosphate to keep 1177 01:19:57,950 --> 01:20:00,820 running this as a cycle? 1178 01:20:00,820 --> 01:20:07,540 And so, in essence, what we need is we need to take six 5-carbon 1179 01:20:07,540 --> 01:20:09,370 units-- 1180 01:20:09,370 --> 01:20:13,035 so that's a total of 30 carbons. 1181 01:20:15,570 --> 01:20:19,980 And we're going to have 6x CO2 molecules. 1182 01:20:19,980 --> 01:20:23,820 There's another six carbons. 1183 01:20:23,820 --> 01:20:29,473 So that's a total of 36 carbons in. 1184 01:20:29,473 --> 01:20:31,140 And then we're going to have to allocate 1185 01:20:31,140 --> 01:20:36,240 those carbons to regenerate six 5-carbon units. 1186 01:20:36,240 --> 01:20:43,560 So the 30 carbons, we need it for the cycle, as well as 1187 01:20:43,560 --> 01:20:49,290 one glucose, the remaining six carbons, that hexose. 1188 01:20:52,030 --> 01:20:54,430 How this happens is confusing. 1189 01:20:54,430 --> 01:20:56,050 I'm sorry, I didn't invent this. 1190 01:20:56,050 --> 01:20:58,390 Nature invented this. 1191 01:20:58,390 --> 01:21:02,860 But it's essentially accomplished via series 1192 01:21:02,860 --> 01:21:06,970 of 2-carbon and 3-carbon swaps. 1193 01:21:06,970 --> 01:21:10,720 And why it's 2-carbon and 3-carbon swaps and not-- 1194 01:21:10,720 --> 01:21:12,880 I don't know, why didn't nature just come up 1195 01:21:12,880 --> 01:21:14,620 with something more straightforward 1196 01:21:14,620 --> 01:21:17,500 is well grounded in the chemistry of how 1197 01:21:17,500 --> 01:21:18,670 these reactions happen. 1198 01:21:18,670 --> 01:21:22,000 We'll describe the chemistry of the swapping reactions 1199 01:21:22,000 --> 01:21:23,920 in great detail next time. 1200 01:21:23,920 --> 01:21:28,210 You'll see that the swaps occur between aldosis and ketosis. 1201 01:21:28,210 --> 01:21:32,592 And so there's actually good evolutionary reasons. 1202 01:21:32,592 --> 01:21:35,050 You'll see that those reactions will be analogous to things 1203 01:21:35,050 --> 01:21:36,700 that we've seen before. 1204 01:21:36,700 --> 01:21:40,150 And effectively, nature, rather than making it easy 1205 01:21:40,150 --> 01:21:42,190 for you to memorize things, came up 1206 01:21:42,190 --> 01:21:45,670 with a way that actually fit what 1207 01:21:45,670 --> 01:21:48,490 happens that enables carbon rearrangement in a way that 1208 01:21:48,490 --> 01:21:51,730 will make this pathway work. 1209 01:21:51,730 --> 01:21:58,090 And so before I go into the details of exactly how this 1210 01:21:58,090 --> 01:22:00,550 happens, I just want to lay out for you 1211 01:22:00,550 --> 01:22:06,340 at a very, very high level how those swaps can 1212 01:22:06,340 --> 01:22:09,850 work to accomplish this goal. 1213 01:22:15,450 --> 01:22:21,640 So basically, If we're going to start with five carbons 1214 01:22:21,640 --> 01:22:27,840 and we're going to add CO2, what's that going to generate? 1215 01:22:27,840 --> 01:22:31,560 Well, we're going to generate, by the RuBisCO reaction, 1216 01:22:31,560 --> 01:22:33,730 3-carbon units. 1217 01:22:33,730 --> 01:22:40,020 So if I start with 6 of these and 6 of these, 1218 01:22:40,020 --> 01:22:46,390 I can generate 12 3-carbon units. 1219 01:22:46,390 --> 01:22:50,650 I can take of those 12 3-carbon units 1220 01:22:50,650 --> 01:22:54,360 and obviously combine them via the reduction 1221 01:22:54,360 --> 01:22:59,020 reactions, gluconeogenesis, ways to generate 6-carbon units. 1222 01:22:59,020 --> 01:23:06,550 And that ultimately is going to be the glucose that I get out. 1223 01:23:06,550 --> 01:23:11,290 So it turns out if I do this reaction five times-- 1224 01:23:11,290 --> 01:23:14,590 so that's 10 3-carbon units-- 1225 01:23:14,590 --> 01:23:16,420 but if I take some of-- so that means 1226 01:23:16,420 --> 01:23:21,520 I'm going to get in the end five 6-carbon units. 1227 01:23:21,520 --> 01:23:26,150 Well, if I use two of those units 1228 01:23:26,150 --> 01:23:28,820 and I do a swap with the remaining two 1229 01:23:28,820 --> 01:23:30,440 of these 3-carbon units-- that is, 1230 01:23:30,440 --> 01:23:33,200 I transfer two carbons from here to there-- 1231 01:23:33,200 --> 01:23:34,170 what am I going to get? 1232 01:23:34,170 --> 01:23:38,850 Well, if I-- that means two carbons. 1233 01:23:38,850 --> 01:23:42,600 6 minus 2 is I get a 4-carbon unit. 1234 01:23:42,600 --> 01:23:44,730 3 plus 2 is a 5-carbon unit. 1235 01:23:44,730 --> 01:23:46,020 That's good. 1236 01:23:46,020 --> 01:23:47,430 That's what I'm trying to get. 1237 01:23:47,430 --> 01:23:50,040 So I took two there, two here. 1238 01:23:50,040 --> 01:23:51,960 So that stoichiometry adds up here. 1239 01:23:51,960 --> 01:23:53,700 I had 12 of these made. 1240 01:23:53,700 --> 01:23:56,280 5 plus 5 plus 2 equals 10, 5 of them 1241 01:23:56,280 --> 01:23:59,730 down here to these guys, 2 of them 1242 01:23:59,730 --> 01:24:01,990 with this swap with the 3-carbon units. 1243 01:24:01,990 --> 01:24:06,810 Now I end up getting two of the six 5-carbon units 1244 01:24:06,810 --> 01:24:08,460 that I need to make. 1245 01:24:08,460 --> 01:24:11,760 Well, now I'm left here with two 4-carbon units. 1246 01:24:11,760 --> 01:24:12,720 I steal here. 1247 01:24:12,720 --> 01:24:15,450 I took one away here, two away there. 1248 01:24:15,450 --> 01:24:18,750 5 minus 2 minus 1 is 2 left, so I still 1249 01:24:18,750 --> 01:24:22,800 have two 2-carbon units here to go. 1250 01:24:25,320 --> 01:24:28,890 So I can use my remaining 2-carbon units 1251 01:24:28,890 --> 01:24:31,080 to react with those 4-carbon units. 1252 01:24:31,080 --> 01:24:38,490 If I now move three carbons from here to there, 6 minus 3 is 3, 1253 01:24:38,490 --> 01:24:40,590 4 plus 3 is 7. 1254 01:24:40,590 --> 01:24:46,260 Now I generate a 7-carbon unit, two of these, two of these. 1255 01:24:46,260 --> 01:24:48,990 Now, if I take two carbons from my 7-carbon unit, 1256 01:24:48,990 --> 01:24:51,690 give it to the 3-carbon unit, what do I have? 1257 01:24:51,690 --> 01:24:53,550 Well, now I have-- 1258 01:24:53,550 --> 01:24:56,985 I've made two different 5-carbon units. 1259 01:25:01,630 --> 01:25:07,300 So 2 plus 2 plus 2 equals 6 5-carbon units back. 1260 01:25:07,300 --> 01:25:11,710 So it can work. 1261 01:25:11,710 --> 01:25:16,490 I will end today by drawing out the details of how it works. 1262 01:25:16,490 --> 01:25:18,550 And then I will redraw them next time, 1263 01:25:18,550 --> 01:25:23,320 and we'll go through in great detail how it works. 1264 01:25:23,320 --> 01:25:44,490 So let's start here with our ribulose 1,5-bisphosphate, 1265 01:25:44,490 --> 01:25:46,980 carry out the RuBisCO reaction. 1266 01:25:46,980 --> 01:25:50,730 I get two 3PG molecules. 1267 01:25:50,730 --> 01:25:59,760 I can run those 3PG molecules using ATP and NADPH, 1268 01:25:59,760 --> 01:26:19,200 ultimately to generate 3-phosphoglycerate molecules. 1269 01:26:19,200 --> 01:26:22,850 I can isomerize-- 1270 01:26:22,850 --> 01:26:25,700 I'm sorry, glyceraldehyde 3-phosphate molecules-- 1271 01:26:25,700 --> 01:26:29,480 I can isomerize those to also generate 1272 01:26:29,480 --> 01:26:34,850 dihydroxyacetone phosphate molecules. 1273 01:26:39,650 --> 01:26:44,510 And, of course, this is what I need 1274 01:26:44,510 --> 01:26:58,400 to generate FBP, and ultimately generate fructose 6-phosphate. 1275 01:27:07,660 --> 01:27:09,550 And that fructose 6-phosphate, of course, 1276 01:27:09,550 --> 01:27:12,160 can be used to generate glucose. 1277 01:27:12,160 --> 01:27:14,720 And I'm not going to draw the steps for it. 1278 01:27:14,720 --> 01:27:15,430 OK. 1279 01:27:15,430 --> 01:27:17,560 All stuff you know. 1280 01:27:17,560 --> 01:27:21,100 RuBisCO reaction and then gluconeogenesis, ultimately, 1281 01:27:21,100 --> 01:27:22,550 to get glucose. 1282 01:27:22,550 --> 01:27:23,050 OK. 1283 01:27:23,050 --> 01:27:24,633 Now, here's where it gets interesting. 1284 01:27:24,633 --> 01:27:28,960 So now let's use our fructose 6-phosphate 1285 01:27:28,960 --> 01:27:32,890 and our glyceraldehyde 3-phosphate. 1286 01:27:32,890 --> 01:27:43,420 So I can generate a 5-carbon sugar 1287 01:27:43,420 --> 01:27:57,820 called xylulose 5-phosphate. 1288 01:27:57,820 --> 01:27:59,950 That's a five-carbon sugar. 1289 01:27:59,950 --> 01:28:09,160 If I use an epimerase and phosphorylate it with ATP, 1290 01:28:09,160 --> 01:28:12,604 now I can regenerate a ribulose 1,5-bisphosphate. 1291 01:28:20,980 --> 01:28:30,990 I've also now will generate this 4-carbon sugar 1292 01:28:30,990 --> 01:28:38,690 called erythrose 4-phosphate. 1293 01:28:38,690 --> 01:28:42,800 And I can take this erythrose 4-phosphate, 1294 01:28:42,800 --> 01:28:45,290 swap three carbons, which will give me 1295 01:28:45,290 --> 01:29:09,730 the 7-carbon sugar called sedoheptulose 7-phosphate, as 1296 01:29:09,730 --> 01:29:22,300 well as a 3-carbon glyceraldehyde 3-phosphate. 1297 01:29:22,300 --> 01:29:26,860 Now I can swap two carbons again, 1298 01:29:26,860 --> 01:29:41,650 and that gives me a ribose 5-phosphate and another-- 1299 01:29:41,650 --> 01:29:46,360 so this is ribose 5-phosphate-- 1300 01:29:46,360 --> 01:29:55,200 and another xylulose 5-phosphate. 1301 01:30:03,920 --> 01:30:07,670 So X5P, xylulose 5-phosphate. 1302 01:30:07,670 --> 01:30:11,510 I can now do exactly the same thing I did before 1303 01:30:11,510 --> 01:30:13,490 with the xylulose 5-phosphate where 1304 01:30:13,490 --> 01:30:18,760 I do in an epimerase reaction and phosphorylate it, 1305 01:30:18,760 --> 01:30:24,910 so epimerase from the ribose 5-phosphate. 1306 01:30:24,910 --> 01:30:32,240 I can do and isomerase reaction and phosphorylate it. 1307 01:30:32,240 --> 01:30:37,915 And in the end, I now generate 5-carbon ribulose 1308 01:30:37,915 --> 01:30:40,170 1,5-bisphosphate. 1309 01:30:40,170 --> 01:30:47,490 So if I start with six ribulose 1,5-bisphosphates and six CO2 1310 01:30:47,490 --> 01:30:49,890 molecules, what do I end up with? 1311 01:30:49,890 --> 01:30:55,850 Well, now I end up with 12 glyceraldehyde 3-phosphates. 1312 01:30:55,850 --> 01:30:59,960 If I allocate two of them here, five of them here, 1313 01:30:59,960 --> 01:31:02,150 and five of them here, that allows 1314 01:31:02,150 --> 01:31:09,410 me to generate five fructose 6-phosphate molecules. 1315 01:31:09,410 --> 01:31:12,820 If I allocate two of them there, pull one of them 1316 01:31:12,820 --> 01:31:16,870 out as glucose, and the remaining two here, 1317 01:31:16,870 --> 01:31:22,645 I get two of these and two of these, can carry this through. 1318 01:31:22,645 --> 01:31:24,020 Two of these and two of those, is 1319 01:31:24,020 --> 01:31:25,940 two of these and two of those, two of these and two of those, 1320 01:31:25,940 --> 01:31:27,980 so I end up with two here, two here. 1321 01:31:27,980 --> 01:31:32,660 That's two, four, six 5-carbon units ending up as ribulose 1322 01:31:32,660 --> 01:31:34,290 1,5-bisphosphate. 1323 01:31:34,290 --> 01:31:39,230 And so I can feed six CO2's in, get a glucose out, 1324 01:31:39,230 --> 01:31:44,720 and net regenerate six ribulose 1,5-bisphosphates, 1325 01:31:44,720 --> 01:31:48,710 all at the cost of six ATPs there, 1326 01:31:48,710 --> 01:31:56,150 another twelve ATPs here to do the gluconeogenesis reactions, 1327 01:31:56,150 --> 01:31:59,270 and another six-- 1328 01:31:59,270 --> 01:32:02,360 I'm sorry, twelve NADPH's to generate 1329 01:32:02,360 --> 01:32:04,170 all of these 3-carbon units. 1330 01:32:04,170 --> 01:32:07,743 And in the end, pull out one glucose molecule. 1331 01:32:07,743 --> 01:32:08,660 We'll start with this. 1332 01:32:08,660 --> 01:32:10,490 I know it's very confusing. 1333 01:32:10,490 --> 01:32:11,810 I will redraw that out. 1334 01:32:11,810 --> 01:32:15,420 I'll go through it again at the start of the next lecture. 1335 01:32:15,420 --> 01:32:17,140 Thank you.