1 00:00:00,000 --> 00:00:02,420 [SQUEAKING][RUSTLING][CLICKING] 2 00:00:09,680 --> 00:00:11,560 MATTHEW VANDER HEIDEN: Last time, we 3 00:00:11,560 --> 00:00:15,220 discussed oxidative phosphorylation, which 4 00:00:15,220 --> 00:00:20,710 is how to couple the NADH generated from sugar, 5 00:00:20,710 --> 00:00:25,390 fatty acids, et cetera; the oxidation of carbon as a way 6 00:00:25,390 --> 00:00:29,200 to carry out favorable electron transfer to oxygen 7 00:00:29,200 --> 00:00:32,650 and use that energy release to charge a battery-- 8 00:00:32,650 --> 00:00:35,890 that is, create this delta psi/delta 9 00:00:35,890 --> 00:00:41,360 pH that can subsequently be used by the battery to do work, 10 00:00:41,360 --> 00:00:44,230 including the synthesis of ADP. 11 00:00:44,230 --> 00:00:48,640 And of course, this can occur at a physiological ATP/ADP ratio 12 00:00:48,640 --> 00:00:51,620 and, therefore, allow the cell to couple ATP 13 00:00:51,620 --> 00:00:54,220 to ADP conversion to other unfavorable processes 14 00:00:54,220 --> 00:00:55,760 in the cell. 15 00:00:55,760 --> 00:00:58,450 Now, all of these reactions occur at the mitochondria. 16 00:00:58,450 --> 00:01:01,510 And I drew, here, a schematic mitochondria for us 17 00:01:01,510 --> 00:01:02,570 just to start. 18 00:01:02,570 --> 00:01:06,290 Remember, the mitochondria has its own genome. 19 00:01:06,290 --> 00:01:10,180 This is a vestige of the symbiotic relationship that 20 00:01:10,180 --> 00:01:15,580 developed long, long ago, where two prokaryotes came together, 21 00:01:15,580 --> 00:01:20,720 leading to this symbiotic relationship-- but of course, 22 00:01:20,720 --> 00:01:23,170 the mitochondrial DNA encoding many 23 00:01:23,170 --> 00:01:27,310 of the key components of this electron transport chain 24 00:01:27,310 --> 00:01:31,480 and ATP synthesis machinery that remains in the mitochondria 25 00:01:31,480 --> 00:01:32,530 today. 26 00:01:32,530 --> 00:01:36,370 Now, ultimately, this process involves the reoxidation 27 00:01:36,370 --> 00:01:43,060 of NADH back to NAD that occurs, that complex I of the electron 28 00:01:43,060 --> 00:01:44,410 transport chain. 29 00:01:44,410 --> 00:01:47,230 Those electrons are transferred into the Q pool, which 30 00:01:47,230 --> 00:01:51,160 then go into complex III to cytochrome c to complex IV, 31 00:01:51,160 --> 00:01:57,130 ultimately being transferred to oxygen as the final electron 32 00:01:57,130 --> 00:01:57,850 acceptor. 33 00:01:57,850 --> 00:02:01,300 And it's this process of favorable electron transport-- 34 00:02:01,300 --> 00:02:03,670 that is, electrons moving from a lower 35 00:02:03,670 --> 00:02:06,190 to a higher standard reduction potential-- that 36 00:02:06,190 --> 00:02:08,380 is coupled to the pumping of protons 37 00:02:08,380 --> 00:02:11,830 and the generation of this delta psi/delta pH. 38 00:02:11,830 --> 00:02:15,460 Now, we also discussed complex II of the electron transport 39 00:02:15,460 --> 00:02:19,120 chain, which is identical to succinate dehydrogenase 40 00:02:19,120 --> 00:02:20,500 in the TCA cycle. 41 00:02:20,500 --> 00:02:26,020 This is involving that FAD/FADH2, which can, itself, 42 00:02:26,020 --> 00:02:28,600 also transfer electrons to the Q pool, 43 00:02:28,600 --> 00:02:30,940 ultimately passing those to complex III 44 00:02:30,940 --> 00:02:33,850 to cytochrome c to complex IV, again, 45 00:02:33,850 --> 00:02:36,730 with oxygen as electron acceptor, 46 00:02:36,730 --> 00:02:39,430 ultimately forming what is essentially 47 00:02:39,430 --> 00:02:43,300 two different variations on the electron transport 48 00:02:43,300 --> 00:02:47,470 chain as a way to generate delta psi/delta pH. 49 00:02:47,470 --> 00:02:52,090 Now I show here, on the slide, that there's really 50 00:02:52,090 --> 00:02:55,030 many versions of this electron transport chain, 51 00:02:55,030 --> 00:02:57,470 and another one we discussed is shown down here. 52 00:02:57,470 --> 00:03:03,220 So up here is the typical one to regenerate NAD+ using complex I 53 00:03:03,220 --> 00:03:05,770 to the Q pool to III to c to IV. 54 00:03:05,770 --> 00:03:10,990 Here's the one involving complex II, so dehydrogenase reaction 55 00:03:10,990 --> 00:03:15,580 via complex II to the Q pool, complex III, c, IV. 56 00:03:15,580 --> 00:03:19,250 And then there's yet a third one here from fatty acid oxidation. 57 00:03:19,250 --> 00:03:21,880 So this is Acyl-CoA dehydrogenase. 58 00:03:21,880 --> 00:03:25,960 Again, like complex II, involves FAD/FADH2. 59 00:03:25,960 --> 00:03:28,870 This occurs at the membrane, transfers 60 00:03:28,870 --> 00:03:31,990 electrons to the Q pool to III to c to IV, 61 00:03:31,990 --> 00:03:35,150 ultimately reducing oxygen to water. 62 00:03:35,150 --> 00:03:38,920 And so the net effect of these various electron 63 00:03:38,920 --> 00:03:41,530 transport chains were shown up here 64 00:03:41,530 --> 00:03:45,670 is generating this delta psi/delta pH, which 65 00:03:45,670 --> 00:03:48,140 can then be used to do work. 66 00:03:48,140 --> 00:03:51,040 Now, the cool thing is is that this 67 00:03:51,040 --> 00:03:53,410 is occurring, in this case, at the inner mitochondrial 68 00:03:53,410 --> 00:03:54,230 membrane. 69 00:03:54,230 --> 00:03:56,890 And so you can do work in lots of different ways. 70 00:03:56,890 --> 00:03:58,640 It doesn't have to be next to the complex. 71 00:03:58,640 --> 00:04:00,610 You have a charge across the membrane that 72 00:04:00,610 --> 00:04:02,660 can be used different places in space, 73 00:04:02,660 --> 00:04:06,850 such as so-called complex V or the F1F0-ATPase, 74 00:04:06,850 --> 00:04:10,330 which can use dissipation of that proton gradient 75 00:04:10,330 --> 00:04:13,990 to drive that rotational catalysis machine we described 76 00:04:13,990 --> 00:04:17,140 last time and, ultimately, take ADP plus phosphate 77 00:04:17,140 --> 00:04:19,450 and synthesize ATP. 78 00:04:19,450 --> 00:04:21,100 But that's not the only work it can do. 79 00:04:21,100 --> 00:04:23,230 You can uncouple the whole thing. 80 00:04:23,230 --> 00:04:25,540 We discussed these uncoupling proteins, 81 00:04:25,540 --> 00:04:28,120 which, effectively, just let protons leak 82 00:04:28,120 --> 00:04:31,570 back across into the membranes, that short-circuiting 83 00:04:31,570 --> 00:04:32,710 this potential. 84 00:04:32,710 --> 00:04:35,320 And that's a way to generate heat. 85 00:04:35,320 --> 00:04:37,180 Other type of work we can do is we 86 00:04:37,180 --> 00:04:41,200 can use this proton gradient to power 87 00:04:41,200 --> 00:04:43,870 the transport of other ions. 88 00:04:43,870 --> 00:04:47,830 We discussed this happening with ADP/ATP exchange 89 00:04:47,830 --> 00:04:50,470 because, of course, ADP-to-ATP conversion 90 00:04:50,470 --> 00:04:53,830 needed in the cytosol to favor unfavorable processes there. 91 00:04:53,830 --> 00:04:56,120 Need to exchange those in and out of the mitochondria, 92 00:04:56,120 --> 00:04:59,950 so you can regenerate ATP with the ATP synthase machinery. 93 00:04:59,950 --> 00:05:01,630 And yet another thing we talked about is 94 00:05:01,630 --> 00:05:03,680 you can use this as a way to concentrate ions, 95 00:05:03,680 --> 00:05:05,110 such as calcium. 96 00:05:05,110 --> 00:05:07,840 And the mitochondria ends up being a major calcium 97 00:05:07,840 --> 00:05:10,860 store for cells. 98 00:05:10,860 --> 00:05:19,170 Now, we discussed last time that this is great having all 99 00:05:19,170 --> 00:05:21,870 of these oxidation processes-- the TCA cycle, 100 00:05:21,870 --> 00:05:23,490 fatty acid oxidation-- 101 00:05:23,490 --> 00:05:25,230 all occurring in the matrix because it 102 00:05:25,230 --> 00:05:30,180 gives a location for NAD/NADH to happen where 103 00:05:30,180 --> 00:05:34,590 it has easy access to this electron transport machinery 104 00:05:34,590 --> 00:05:38,550 and the ability to regenerate the NAD necessary to keep these 105 00:05:38,550 --> 00:05:40,180 processes going. 106 00:05:40,180 --> 00:05:43,050 But of course, the very first process we talked about happens 107 00:05:43,050 --> 00:05:45,930 in the cytosol, and that's glycolysis. 108 00:05:45,930 --> 00:05:48,240 And we discussed how you can bring pyruvate 109 00:05:48,240 --> 00:05:50,800 into the mitochondria through this mitochondrial pyruvate 110 00:05:50,800 --> 00:05:51,840 carrier. 111 00:05:51,840 --> 00:05:54,660 Then it has access to the pyruvate dehydrogenase 112 00:05:54,660 --> 00:05:57,800 reaction, which, of course, generates acetyl-CoA 113 00:05:57,800 --> 00:06:00,310 and NADH in the right place. 114 00:06:00,310 --> 00:06:03,360 However, if you go way back to when we discuss fermentation, 115 00:06:03,360 --> 00:06:07,020 remember, the reason we had to send pyruvate to fermentation 116 00:06:07,020 --> 00:06:11,130 was to regenerate that NAD in the cytosol that's 117 00:06:11,130 --> 00:06:14,230 necessary to keep glycolysis going. 118 00:06:14,230 --> 00:06:17,910 Now, of course, if we're going to fully oxidize 119 00:06:17,910 --> 00:06:20,220 pyruvate carbon in the mitochondria, 120 00:06:20,220 --> 00:06:23,340 we still have to solve this electron balance 121 00:06:23,340 --> 00:06:25,500 problem for glycolysis. 122 00:06:25,500 --> 00:06:27,660 And as we discussed, we can solve that 123 00:06:27,660 --> 00:06:29,790 by giving those electrons to oxygen. 124 00:06:29,790 --> 00:06:33,000 But now you see that there's this additional complication-- 125 00:06:33,000 --> 00:06:36,480 is that these electrons in the form of NADH 126 00:06:36,480 --> 00:06:39,420 need to get inside the mitochondrial matrix 127 00:06:39,420 --> 00:06:42,060 or get to the electron transport chain 128 00:06:42,060 --> 00:06:46,740 as a way to use oxygen to ultimately dispose 129 00:06:46,740 --> 00:06:47,800 of those electrons. 130 00:06:47,800 --> 00:06:50,400 And so, effectively, that NADH needs 131 00:06:50,400 --> 00:06:53,430 to be gotten to complex I, which would 132 00:06:53,430 --> 00:06:57,900 be the matrix in the way we've drawn it here. 133 00:06:57,900 --> 00:07:04,450 Now, really, the way that this happens is not direct. 134 00:07:04,450 --> 00:07:09,250 So you don't transport NADH from the cytosol 135 00:07:09,250 --> 00:07:11,590 into the mitochondria. 136 00:07:11,590 --> 00:07:15,910 And that's partially because one of the benefits, remember, 137 00:07:15,910 --> 00:07:18,460 of having compartmentalized metabolism 138 00:07:18,460 --> 00:07:21,260 is your ability to have different conditions; 139 00:07:21,260 --> 00:07:24,467 different ATP/ADP ratios, NAD/NADH ratios 140 00:07:24,467 --> 00:07:26,050 in different compartments because that 141 00:07:26,050 --> 00:07:28,070 can favor different chemistry. 142 00:07:28,070 --> 00:07:33,850 And so the way that you get electrons into the matrix 143 00:07:33,850 --> 00:07:36,850 or get electrons to the electron transport chains 144 00:07:36,850 --> 00:07:38,260 doesn't occur directly. 145 00:07:38,260 --> 00:07:42,370 You do not transport NAD or NADH directly. 146 00:07:42,370 --> 00:07:47,220 Instead, you use so-called shuttles, 147 00:07:47,220 --> 00:07:50,490 which are analogous to some of the other shuttles we've 148 00:07:50,490 --> 00:07:53,640 talked about, in order to get materials across membranes. 149 00:07:53,640 --> 00:07:55,470 In this case, this is a way to get 150 00:07:55,470 --> 00:07:58,350 electrons generated from oxidation 151 00:07:58,350 --> 00:08:00,090 reactions in the cytosol-- 152 00:08:00,090 --> 00:08:02,400 production of NADH in the cytosol, things 153 00:08:02,400 --> 00:08:05,040 like glycolysis-- 154 00:08:05,040 --> 00:08:08,520 to the electron transport chain so that they can ultimately 155 00:08:08,520 --> 00:08:12,120 be transferred to oxygen. 156 00:08:12,120 --> 00:08:20,300 And so these shuttles, or redox shuttles, 157 00:08:20,300 --> 00:08:23,540 actually come in several different ways 158 00:08:23,540 --> 00:08:27,900 that cells can deal with cytosolic electrons. 159 00:08:27,900 --> 00:08:34,230 And so these shuttles also, in essence, 160 00:08:34,230 --> 00:08:38,850 help cells also establish these different conditions, 161 00:08:38,850 --> 00:08:42,560 these different NAD/NADH ratios in different compartments. 162 00:08:42,560 --> 00:08:45,060 And so you can imagine that the shuttles themselves could be 163 00:08:45,060 --> 00:08:47,700 regulated as a way to do this. 164 00:08:47,700 --> 00:08:50,700 Now, there's two major redox shuttles 165 00:08:50,700 --> 00:08:54,390 that allow you to move electrons from the cytosol 166 00:08:54,390 --> 00:09:01,470 into the mitochondria, and we'll discuss those two here next. 167 00:09:01,470 --> 00:09:03,570 These are the two that probably are 168 00:09:03,570 --> 00:09:08,700 being looked for if you get this question on an MCAT exam 169 00:09:08,700 --> 00:09:10,302 or something like that. 170 00:09:10,302 --> 00:09:11,760 They're the ones that are described 171 00:09:11,760 --> 00:09:15,090 in the text as the way to get electrons to the electron 172 00:09:15,090 --> 00:09:16,380 transport chain. 173 00:09:16,380 --> 00:09:18,910 But however, as we go through them, 174 00:09:18,910 --> 00:09:21,730 you will see that there's a couple different ways 175 00:09:21,730 --> 00:09:22,470 one can do this. 176 00:09:22,470 --> 00:09:25,710 They basically involve oxidation and reduction 177 00:09:25,710 --> 00:09:29,670 of a pair of molecules on either side of the membrane. 178 00:09:29,670 --> 00:09:32,160 And there's lots of ways you can do that in addition 179 00:09:32,160 --> 00:09:34,290 to these other shuttles. 180 00:09:34,290 --> 00:09:36,240 It'll become more clear what I mean by this, 181 00:09:36,240 --> 00:09:39,030 but I guess I just want you to know that these are not 182 00:09:39,030 --> 00:09:41,820 the only ways one could shuttle electrons 183 00:09:41,820 --> 00:09:43,260 into the mitochondria. 184 00:09:43,260 --> 00:09:48,450 These are just the major ways that have been discussed 185 00:09:48,450 --> 00:09:52,630 and are usually discussed in textbooks. 186 00:09:52,630 --> 00:09:57,000 So the first shuttle system is something called 187 00:09:57,000 --> 00:10:02,865 the glycerol phosphate shuttle. 188 00:10:05,490 --> 00:10:08,910 And so the glycerol phosphate shuttle 189 00:10:08,910 --> 00:10:13,080 involves a reaction that we've already discussed. 190 00:10:22,750 --> 00:10:26,610 So here's our old friend, dihyroxyacetone phosphate 191 00:10:26,610 --> 00:10:28,920 from glycolysis. 192 00:10:28,920 --> 00:10:32,850 Remember when we talked about the generation of 193 00:10:32,850 --> 00:10:34,770 and metabolism of glycerol, in order 194 00:10:34,770 --> 00:10:37,530 to make triacylglycerides, phospholipids-- 195 00:10:37,530 --> 00:10:40,140 remember we described that you can 196 00:10:40,140 --> 00:10:42,330 take dihyroxyacetone phosphate. 197 00:10:46,540 --> 00:11:02,260 If you use NADH to reduce that ketone to the alcohol, 198 00:11:02,260 --> 00:11:07,580 that generates this molecule, glycerol phosphate. 199 00:11:07,580 --> 00:11:10,130 And of course, because you're reducing the ketone 200 00:11:10,130 --> 00:11:12,470 to the alcohol, something else has to be oxidized. 201 00:11:12,470 --> 00:11:16,430 You reoxidized NADH back to NAD+, 202 00:11:16,430 --> 00:11:21,610 effectively disposing of those electrons from, say, 203 00:11:21,610 --> 00:11:28,150 glycolysis that generated NADH to regenerate NAD+. 204 00:11:28,150 --> 00:11:30,880 Now, rather than use this glycerol phosphate and lipid 205 00:11:30,880 --> 00:11:34,090 synthesis, turns out there's a complex that 206 00:11:34,090 --> 00:11:44,940 can directly transfer those electrons to FAD 207 00:11:44,940 --> 00:11:49,740 to generate FADH2. 208 00:11:49,740 --> 00:11:56,750 And this complex sits within the inner mitochondrial membrane, 209 00:11:56,750 --> 00:12:01,040 facing the cytosolic side of the membrane. 210 00:12:04,232 --> 00:12:06,260 Here's the inner mitochondrial membrane. 211 00:12:06,260 --> 00:12:09,170 This would be the matrix. 212 00:12:09,170 --> 00:12:11,920 And so transferring those electrons 213 00:12:11,920 --> 00:12:17,090 to FAD to reduce to FADH2 reoxidizes 214 00:12:17,090 --> 00:12:20,920 the glycerol phosphate back to dihydroxyacetone phosphate. 215 00:12:20,920 --> 00:12:24,520 And of course, in this membrane, you can, in this complex, 216 00:12:24,520 --> 00:12:28,480 transfer those electrons by oxidizing the FAD 217 00:12:28,480 --> 00:12:33,070 and sending them to the coenzyme Q pool. 218 00:12:33,070 --> 00:12:36,370 That can take the ubiquinone, makes ubinquinol. 219 00:12:36,370 --> 00:12:38,410 Those electrons can go to complex III. 220 00:12:38,410 --> 00:12:42,790 That can go to cytochrome C. They can go to complex IV, 221 00:12:42,790 --> 00:12:51,070 ultimately ending up reducing oxygen to water using oxygen 222 00:12:51,070 --> 00:12:53,260 as a final electron acceptor. 223 00:12:53,260 --> 00:12:58,330 Again, this is effectively another alternative 224 00:12:58,330 --> 00:13:00,620 to the electron transport chain. 225 00:13:00,620 --> 00:13:02,340 And so I show that here on the slide. 226 00:13:02,340 --> 00:13:03,630 And so we've discussed-- 227 00:13:08,830 --> 00:13:13,750 the complex II-- this is the FAD/FADH2 containing enzyme 228 00:13:13,750 --> 00:13:14,780 is complex II. 229 00:13:14,780 --> 00:13:18,190 Succinate dehydrogenase sits in the inner mitochondrial 230 00:13:18,190 --> 00:13:21,160 membrane, converts succinate to fumarate. 231 00:13:21,160 --> 00:13:24,280 Those electrons end up on up FADH2, 232 00:13:24,280 --> 00:13:25,725 can get transferred to the Q pool, 233 00:13:25,725 --> 00:13:31,420 reoxidizing FAD to complex III to cytochrome C to complex IV 234 00:13:31,420 --> 00:13:34,030 to oxygen to water. 235 00:13:34,030 --> 00:13:37,540 We already discussed acyl-CoA dehydrogenase-- same thing. 236 00:13:37,540 --> 00:13:41,290 FADH2 containing reaction in there is also in the membrane-- 237 00:13:41,290 --> 00:13:44,200 ultimately allows another version electron transport 238 00:13:44,200 --> 00:13:44,980 chain. 239 00:13:44,980 --> 00:13:49,240 And here's yet a third one involving an FDA using enzyme. 240 00:13:49,240 --> 00:13:53,860 The difference is is that these have the oxidation reduction 241 00:13:53,860 --> 00:13:56,110 reaction occurring on the matrix side-- 242 00:13:56,110 --> 00:13:58,900 succinate to fumarate on the matrix side-- 243 00:13:58,900 --> 00:14:01,090 the introduction of a double bond 244 00:14:01,090 --> 00:14:03,610 for fatty acid oxidation and the matrix side. 245 00:14:03,610 --> 00:14:07,120 This one here instead faces the cytosolic side. 246 00:14:07,120 --> 00:14:12,130 So glycerol 3 phosphate can be reoxidized 247 00:14:12,130 --> 00:14:17,220 to dihydroxyacetone phosphate-- 248 00:14:17,220 --> 00:14:19,970 looks like I have those arrows drawn backwards, sorry-- 249 00:14:19,970 --> 00:14:25,010 ultimately sending electrons into this glycerol phosphate 250 00:14:25,010 --> 00:14:29,890 shuttle, reducing FADH2, which can be reoxidized 251 00:14:29,890 --> 00:14:33,170 by giving those electrons to the Q pool to complex III, 252 00:14:33,170 --> 00:14:36,400 to cytochrome C, to complex IV, ultimately 253 00:14:36,400 --> 00:14:40,900 with water serving as the final electron acceptor, 254 00:14:40,900 --> 00:14:43,460 as I've drawn here. 255 00:14:43,460 --> 00:14:48,980 Now, if you notice, if we use this system or any 256 00:14:48,980 --> 00:14:54,680 of these systems, these alternative FAD/FADH2 using 257 00:14:54,680 --> 00:14:59,390 electron transport chains, you don't use complex I. 258 00:14:59,390 --> 00:15:05,250 Remember, complex I will pump a proton, complex II will not. 259 00:15:05,250 --> 00:15:10,130 And that means that if you use any of these FADH2 using 260 00:15:10,130 --> 00:15:12,380 alternative electron transport chains, 261 00:15:12,380 --> 00:15:15,740 you pump fewer protons into the cytosol. 262 00:15:15,740 --> 00:15:19,580 And so if you donate the electrons to complex I 263 00:15:19,580 --> 00:15:22,280 from the matrix, you pump a proton 264 00:15:22,280 --> 00:15:24,890 as you go through complex I to the Q pool 265 00:15:24,890 --> 00:15:27,620 if you go through the glycerol phosphate shuttle, 266 00:15:27,620 --> 00:15:31,550 you avoid complex I, because you give those electrons directly 267 00:15:31,550 --> 00:15:33,380 to the glycerol phosphate shuttle. 268 00:15:33,380 --> 00:15:36,890 And FAD/FADH2 enzyme-- those end up in the Q pool. 269 00:15:36,890 --> 00:15:38,960 Less protons are pumped. 270 00:15:38,960 --> 00:15:41,540 And this is why sometimes in textbooks, you'll 271 00:15:41,540 --> 00:15:46,070 see that you get less ATP per NADH produced in the cytosol 272 00:15:46,070 --> 00:15:51,420 than you get for NADH produced in the mitochondria. 273 00:15:51,420 --> 00:15:55,280 And this is one of the ways that you end up 274 00:15:55,280 --> 00:15:59,480 with these accounting yields for glucose oxidation-- something 275 00:15:59,480 --> 00:16:02,430 we'll talk about in a minute. 276 00:16:02,430 --> 00:16:05,390 But the deeper reason for this, which I want you to appreciate, 277 00:16:05,390 --> 00:16:12,180 is because FADH2 has a higher standard reduction potential. 278 00:16:12,180 --> 00:16:20,210 So the standard reduction potential of FAD/FADH2 279 00:16:20,210 --> 00:16:24,245 is greater than it is for NAD/NADH. 280 00:16:27,700 --> 00:16:30,070 In essence, that means that there's 281 00:16:30,070 --> 00:16:31,780 less change in standard reduction 282 00:16:31,780 --> 00:16:37,780 potential for transferring FADH2 electrons to oxygen as there is 283 00:16:37,780 --> 00:16:41,290 for transferring electrons to oxygen. 284 00:16:41,290 --> 00:16:44,950 And so less energy released, less protons pumped, 285 00:16:44,950 --> 00:16:49,600 and so that's why you get less ATP, if you will, 286 00:16:49,600 --> 00:16:54,600 produced going through the glycerol phosphate shuttle. 287 00:16:54,600 --> 00:16:56,450 All right. 288 00:16:56,450 --> 00:17:01,370 Next, I want to discuss another alternative shuttle 289 00:17:01,370 --> 00:17:03,930 to get electrons into the mitochondria. 290 00:17:03,930 --> 00:17:07,310 And this involves much more what I was alluding to earlier. 291 00:17:07,310 --> 00:17:09,500 And that's oxidation and reduction 292 00:17:09,500 --> 00:17:13,045 of a pair of metabolites that are transported 293 00:17:13,045 --> 00:17:15,170 across the membrane on either side of the membrane, 294 00:17:15,170 --> 00:17:19,490 effectively moving electrons from one compartment 295 00:17:19,490 --> 00:17:21,390 in the cell to another. 296 00:17:21,390 --> 00:17:25,755 Now, to understand how this shuttle works, I have to-- 297 00:17:25,755 --> 00:17:27,380 because it's a little more complicated, 298 00:17:27,380 --> 00:17:29,750 I have to first describe that there's 299 00:17:29,750 --> 00:17:34,830 a relationship between amino acids and alpha-keto acids. 300 00:17:34,830 --> 00:17:36,240 And so, what do I mean by that? 301 00:17:36,240 --> 00:17:44,150 Well, here's a alpha-keto acid that we've talked about a lot. 302 00:17:44,150 --> 00:17:47,600 So this is oxaloacetate. 303 00:17:47,600 --> 00:17:51,050 Remember, it's an alpha keto-acid because this ketone 304 00:17:51,050 --> 00:17:56,160 is alpha to this carboxylic acid-- 305 00:17:56,160 --> 00:17:58,860 so an alpha-keto acid. 306 00:17:58,860 --> 00:18:03,200 Turns out that alpha-keto acids are 307 00:18:03,200 --> 00:18:09,930 related to amino acids in the following way. 308 00:18:09,930 --> 00:18:13,550 And so if I take that alpha-keto group 309 00:18:13,550 --> 00:18:19,780 and instead make it an amino group, 310 00:18:19,780 --> 00:18:23,590 this here is the amino acid aspartame. 311 00:18:29,030 --> 00:18:31,440 Now, we'll discuss later in the course 312 00:18:31,440 --> 00:18:33,710 the chemistry that allows you to interconvert 313 00:18:33,710 --> 00:18:36,650 this alpha-keto group with this amino group. 314 00:18:36,650 --> 00:18:40,700 But that's effectively where your amino acids come from-- 315 00:18:40,700 --> 00:18:46,130 how your amino acids are related to other aspects of carbon 316 00:18:46,130 --> 00:18:46,910 metabolism-- 317 00:18:46,910 --> 00:18:50,090 Alpha-keto acids that are generated in metabolism. 318 00:18:50,090 --> 00:18:52,910 And so there's one example-- 319 00:18:52,910 --> 00:18:59,310 oxaloacetate and aspartate, an alpha-keto acid, an amino acid. 320 00:18:59,310 --> 00:19:00,215 Here's another one. 321 00:19:09,290 --> 00:19:11,720 So this is alpha-ketoglutarate-- 322 00:19:16,510 --> 00:19:19,310 an alpha-keto acid. 323 00:19:19,310 --> 00:19:30,770 And if we change that alpha group to an amino group, 324 00:19:30,770 --> 00:19:39,190 now we get the amino acid glutamate. 325 00:19:39,190 --> 00:19:40,120 All right. 326 00:19:40,120 --> 00:19:44,380 To remind you, the three letter abbreviation for aspartate 327 00:19:44,380 --> 00:19:45,400 is a-s-p. 328 00:19:45,400 --> 00:19:49,580 The one letter is D. The three letter abbreviation 329 00:19:49,580 --> 00:19:53,480 for glutamate is g-l-u. 330 00:19:53,480 --> 00:19:56,270 And the one letter abbreviation is E. 331 00:19:56,270 --> 00:20:00,890 And I just remind you of that in case I use those abbreviations. 332 00:20:00,890 --> 00:20:06,200 But effectively, you can note that if you couple these swaps 333 00:20:06,200 --> 00:20:07,190 to each other-- 334 00:20:07,190 --> 00:20:11,450 that is, if I take oxaloacetate and turn it 335 00:20:11,450 --> 00:20:19,620 into aspartate while at the same time taking glutamate 336 00:20:19,620 --> 00:20:23,280 and turning it into alpha-ketoglutarate, 337 00:20:23,280 --> 00:20:27,630 you can see that that is a reaction where 338 00:20:27,630 --> 00:20:29,700 all atoms are conserved. 339 00:20:29,700 --> 00:20:35,970 That is, if I turn this amino acid into this alpha-keto acid 340 00:20:35,970 --> 00:20:38,250 while at the same time turning that alpha-keto acid 341 00:20:38,250 --> 00:20:43,680 into this amino acid, I have not gained or lost any atoms 342 00:20:43,680 --> 00:20:46,710 or molecules in the process. 343 00:20:46,710 --> 00:20:48,780 And we will see later in the course 344 00:20:48,780 --> 00:20:50,850 that these types of interconversions 345 00:20:50,850 --> 00:20:57,090 is exactly how you run these reactions to do 346 00:20:57,090 --> 00:20:59,310 these interconversions between alpha-keto acids 347 00:20:59,310 --> 00:21:01,980 and amino acids. 348 00:21:01,980 --> 00:21:02,910 OK. 349 00:21:02,910 --> 00:21:04,410 Why am I telling you this now? 350 00:21:04,410 --> 00:21:07,200 Because it turns out this is central to understand 351 00:21:07,200 --> 00:21:11,850 the other major shuttle, which is referred 352 00:21:11,850 --> 00:21:22,500 to as the malate-aspartate shuttle as another way 353 00:21:22,500 --> 00:21:26,730 to get electrons and NADH from the cytosol 354 00:21:26,730 --> 00:21:29,300 into the mitochondria. 355 00:21:29,300 --> 00:21:29,800 All right. 356 00:21:29,800 --> 00:21:31,370 So how does this happen? 357 00:21:31,370 --> 00:21:35,060 So here if you have-- 358 00:21:40,770 --> 00:21:43,260 here's oxaloacetate. 359 00:21:43,260 --> 00:21:48,270 This is occurring up here in the cytosol. 360 00:21:48,270 --> 00:21:49,320 OK. 361 00:21:49,320 --> 00:21:55,140 And if we take oxaloacetate in the cytosol and we 362 00:21:55,140 --> 00:22:01,490 utilize the malate dehydrogenase reaction 363 00:22:01,490 --> 00:22:03,755 that we heard about from the TCA cycle-- 364 00:22:12,080 --> 00:22:15,590 so basically this is the reverse of the malate dehydrogenase 365 00:22:15,590 --> 00:22:17,960 reaction we discussed in the TCA cycle. 366 00:22:17,960 --> 00:22:21,470 We're going to reduce this ketone to the alcohol. 367 00:22:21,470 --> 00:22:25,430 NADH is reoxidized to NAD+. 368 00:22:25,430 --> 00:22:38,950 That will generate malate. 369 00:22:38,950 --> 00:22:45,820 And now we've regenerated NAD+ in the cytosol by putting those 370 00:22:45,820 --> 00:22:50,605 electrons onto oxaloacetate to make malate. 371 00:22:50,605 --> 00:22:52,060 All right. 372 00:22:52,060 --> 00:22:57,610 Now, we can take that malate, send it 373 00:22:57,610 --> 00:23:10,360 across the mitochondrial membranes via transporter 374 00:23:10,360 --> 00:23:15,470 to generate, on the matrix side of the membrane, 375 00:23:15,470 --> 00:23:25,620 malate, whereas from the TCA cycle, 376 00:23:25,620 --> 00:23:40,920 we also have malate dehydrogenase that can now 377 00:23:40,920 --> 00:23:52,780 reoxidize that carbon from the alcohol to the ketone 378 00:23:52,780 --> 00:23:58,000 and regenerate oxaloacetate effectively 379 00:23:58,000 --> 00:24:02,320 by carrying out the same reaction through a redox pair 380 00:24:02,320 --> 00:24:03,850 on two sides of the membrane. 381 00:24:03,850 --> 00:24:05,900 Effectively what I've done is I've 382 00:24:05,900 --> 00:24:09,340 moved the electrons from NADH in the cytosol 383 00:24:09,340 --> 00:24:13,780 to now being NADH in the mitochondrial matrix, where 384 00:24:13,780 --> 00:24:16,510 this can now donate the electrons to complex I, 385 00:24:16,510 --> 00:24:21,460 to the Q pool, to complex III, to cytochrome C, to complex IV, 386 00:24:21,460 --> 00:24:25,300 and ultimately to oxygen, just like it 387 00:24:25,300 --> 00:24:29,350 would for the TCA cycle. 388 00:24:29,350 --> 00:24:29,850 All right. 389 00:24:29,850 --> 00:24:31,990 Now, the simplest thing, of course, 390 00:24:31,990 --> 00:24:34,470 would just be to send the oxaloacetate back out 391 00:24:34,470 --> 00:24:36,480 to the cytosol. 392 00:24:36,480 --> 00:24:38,400 But that's not what happens. 393 00:24:38,400 --> 00:24:42,600 Instead, what happens is you turn that oxaloacetate 394 00:24:42,600 --> 00:24:45,460 into aspartate. 395 00:24:45,460 --> 00:24:47,080 This is an alpha-keto acid. 396 00:24:47,080 --> 00:24:49,360 Turn it into the amino acid. 397 00:24:49,360 --> 00:24:56,130 If you do that, you basically at the same time 398 00:24:56,130 --> 00:25:01,890 turn glutamate into alpha-ketoglutarate all right. 399 00:25:01,890 --> 00:25:05,170 Turns out it's that alpha-ketoglutarate 400 00:25:05,170 --> 00:25:10,890 that is exchanged when you bring malate into the cell. 401 00:25:10,890 --> 00:25:15,720 So malate is exchanged for alpha-ketoglutarate. 402 00:25:15,720 --> 00:25:41,320 And at the same time, you exchange the aspartate 403 00:25:41,320 --> 00:25:44,560 for a glutamate. 404 00:26:00,390 --> 00:26:01,470 All right. 405 00:26:01,470 --> 00:26:05,700 So now you can take that aspartame, 406 00:26:05,700 --> 00:26:09,360 turn it back into oxaloacetate. 407 00:26:09,360 --> 00:26:12,720 To do so you need to balance this. 408 00:26:12,720 --> 00:26:19,550 So you return your alpha-ketoglutarate back 409 00:26:19,550 --> 00:26:21,620 into glutamate. 410 00:26:21,620 --> 00:26:32,960 And effectively, this is the malate aspartate shuttle. 411 00:26:32,960 --> 00:26:33,530 OK. 412 00:26:33,530 --> 00:26:41,590 So electrons from oxaloacetate to malate, and then malate back 413 00:26:41,590 --> 00:26:44,770 to oxaloacetate moves the electrons on NADH 414 00:26:44,770 --> 00:26:47,890 from the cytosol into the mitochondria. 415 00:26:47,890 --> 00:26:49,960 To get that malate across the membrane, 416 00:26:49,960 --> 00:26:52,390 you exchange it for alpha-ketoglutarate 417 00:26:52,390 --> 00:26:55,480 and to maintain carbon balance across the-- 418 00:26:55,480 --> 00:26:58,030 and nitrogen balance-- across the whole thing. 419 00:26:58,030 --> 00:27:01,840 At the same time, you also exchange an aspartate 420 00:27:01,840 --> 00:27:10,170 for a glutamate, which allows you to basically interconvert 421 00:27:10,170 --> 00:27:13,380 the oxaloacetate and the aspartate 422 00:27:13,380 --> 00:27:16,650 on either side of the membrane via interconverting 423 00:27:16,650 --> 00:27:19,440 glutamate and alpha-ketoglutarate. 424 00:27:19,440 --> 00:27:23,910 So somewhat confusing systems, but 425 00:27:23,910 --> 00:27:27,840 recognize that the net effect is moving NADH 426 00:27:27,840 --> 00:27:30,490 from one side of the membrane to the other. 427 00:27:30,490 --> 00:27:34,740 And that occurs because you're interconverting to redox pairs 428 00:27:34,740 --> 00:27:38,250 in opposite directions-- in this case oxaloacetate and malate-- 429 00:27:38,250 --> 00:27:40,860 on either side of the membrane. 430 00:27:40,860 --> 00:27:44,710 And that's why what I alluded to earlier, 431 00:27:44,710 --> 00:27:47,820 you can imagine that you could easily come up 432 00:27:47,820 --> 00:27:50,430 with other shuttle systems where you have a redox 433 00:27:50,430 --> 00:27:52,410 reaction on one side of the membrane 434 00:27:52,410 --> 00:27:54,810 and the reverse of that redox reaction on the other side 435 00:27:54,810 --> 00:27:55,800 of the membrane. 436 00:27:55,800 --> 00:27:58,440 And as long as you can maintain carbon balance, 437 00:27:58,440 --> 00:28:02,580 the net effect is moving electrons across the membranes 438 00:28:02,580 --> 00:28:04,920 from one compartment to another. 439 00:28:04,920 --> 00:28:09,810 And that is a way to allow you to generate NADH that's 440 00:28:09,810 --> 00:28:11,760 now in the matrix, which has access 441 00:28:11,760 --> 00:28:15,210 to complex I of the electron transport chain 442 00:28:15,210 --> 00:28:20,230 as we discussed now several time. 443 00:28:20,230 --> 00:28:21,130 All right. 444 00:28:21,130 --> 00:28:27,010 Now we're ready to go through and discuss the accounting 445 00:28:27,010 --> 00:28:29,320 that you see-- 446 00:28:33,390 --> 00:28:38,130 the accounting that comes up all the time that is in textbooks 447 00:28:38,130 --> 00:28:41,160 and perhaps you memorized from previous classes 448 00:28:41,160 --> 00:28:46,560 about how the ATP yield works for different pathways. 449 00:28:46,560 --> 00:28:48,540 So hopefully it's clear to you by now 450 00:28:48,540 --> 00:28:52,450 that these numbers that you get are estimates. 451 00:28:52,450 --> 00:28:55,830 And that's because there is no direct relationship 452 00:28:55,830 --> 00:29:00,880 between NADH or FADH to NATP. 453 00:29:00,880 --> 00:29:04,840 These interconversions occur via oxidative phosphorylation, 454 00:29:04,840 --> 00:29:08,800 these electron transport chains, delta psi/delta pH. 455 00:29:08,800 --> 00:29:10,900 And so all of these estimates take 456 00:29:10,900 --> 00:29:15,640 into account various assumptions about how efficient 457 00:29:15,640 --> 00:29:18,820 the electron transport chain is, oxidative phosphorylation 458 00:29:18,820 --> 00:29:23,650 works, bounded, of course, by the change 459 00:29:23,650 --> 00:29:27,640 in standard reduction potential, what's the free energy released 460 00:29:27,640 --> 00:29:28,510 from that? 461 00:29:28,510 --> 00:29:33,190 What's the free energy to synthesize ADP to ATP. 462 00:29:33,190 --> 00:29:35,740 Of course, those require assumptions. 463 00:29:35,740 --> 00:29:40,420 And it's, in the end, why if you read different textbooks 464 00:29:40,420 --> 00:29:43,120 and different sources you will find numbers 465 00:29:43,120 --> 00:29:44,810 that are actually a range. 466 00:29:44,810 --> 00:29:49,420 There is no fixed number for what the equivalent is 467 00:29:49,420 --> 00:29:51,760 of an ATP for an NADH. 468 00:29:51,760 --> 00:29:54,950 And that's because it really depends on several variables. 469 00:29:54,950 --> 00:29:58,510 So let's go through what those variables are, 470 00:29:58,510 --> 00:29:59,950 because if we can understand that, 471 00:29:59,950 --> 00:30:03,450 you really understand how this system works. 472 00:30:03,450 --> 00:30:05,420 So the first variable-- 473 00:30:05,420 --> 00:30:08,210 at least if we're talking about something like glucose, 474 00:30:08,210 --> 00:30:12,180 where we generate NADH in the cytosol, 475 00:30:12,180 --> 00:30:17,090 we have to consider the redox shuttles. 476 00:30:17,090 --> 00:30:21,110 And hopefully you appreciate now that you have this need 477 00:30:21,110 --> 00:30:24,560 to regenerate NAD in the cytosol to keep glycolysis going. 478 00:30:24,560 --> 00:30:27,050 You got to get those electrons somehow to the electron 479 00:30:27,050 --> 00:30:28,310 transport chain. 480 00:30:28,310 --> 00:30:32,750 And I just described for you two different ways you can do that. 481 00:30:32,750 --> 00:30:34,970 Now, if I use the glycerol phosphate shuttle, 482 00:30:34,970 --> 00:30:37,860 I'm skipping complex I altogether. 483 00:30:37,860 --> 00:30:40,040 And so that's not pumping any protons. 484 00:30:40,040 --> 00:30:42,620 If I'm using the malate aspartate shuttle, 485 00:30:42,620 --> 00:30:45,620 well, I have all the energetics of doing this gymnastics. 486 00:30:45,620 --> 00:30:50,210 But in the end, now I can use complex I and pump protons. 487 00:30:50,210 --> 00:30:54,710 And so the yield of ATP, if you will-- how many 488 00:30:54,710 --> 00:30:59,180 protons I can pump to generate delta psi/delta pH will 489 00:30:59,180 --> 00:31:01,370 be different if I use the glycerol phosphate 490 00:31:01,370 --> 00:31:06,370 shuttle or a shuttle like the malate aspartate shuttle. 491 00:31:06,370 --> 00:31:09,400 The second variable in consideration 492 00:31:09,400 --> 00:31:15,550 is, what is the efficiency of proton 493 00:31:15,550 --> 00:31:24,670 pumping by the various electron transport chain ETC complexes? 494 00:31:29,310 --> 00:31:33,040 That's not so straightforward and, of course, 495 00:31:33,040 --> 00:31:35,850 is going to depend on a couple of things. 496 00:31:35,850 --> 00:31:39,660 Remember, we spent a long time talking about ATP/ADP, 497 00:31:39,660 --> 00:31:43,260 and how it's the ratio of ATP/ADP that was really 498 00:31:43,260 --> 00:31:45,180 the energy there, because, remember, 499 00:31:45,180 --> 00:31:49,110 thermodynamics is dependent on that log 500 00:31:49,110 --> 00:31:52,800 of the reactants over the product's term in the delta G 501 00:31:52,800 --> 00:31:54,220 equation. 502 00:31:54,220 --> 00:31:58,500 Well, that's true also for the NAD/NADH ratio. 503 00:31:58,500 --> 00:32:01,650 And it's also true for how high the membrane potential is. 504 00:32:01,650 --> 00:32:04,200 And so how efficient this pumping 505 00:32:04,200 --> 00:32:06,930 is going to be is going to be depending on what 506 00:32:06,930 --> 00:32:10,800 is the delta psi to begin with, and what is the NAD/NADH ratio 507 00:32:10,800 --> 00:32:15,690 or the FAD/FADH2 ratio, because all of those things 508 00:32:15,690 --> 00:32:18,150 will affect how much free energy there 509 00:32:18,150 --> 00:32:21,930 is to be able to move protons from one side of the membrane 510 00:32:21,930 --> 00:32:24,540 to the other. 511 00:32:24,540 --> 00:32:27,180 Once I get those protons across the membrane, 512 00:32:27,180 --> 00:32:30,480 then there's the question of how-- 513 00:32:30,480 --> 00:32:32,010 and for lack of a better word, I'm 514 00:32:32,010 --> 00:32:41,720 going to say tight the membrane is to proton leak. 515 00:32:41,720 --> 00:32:45,200 That is, you can imagine that to make ATP, 516 00:32:45,200 --> 00:32:47,420 I have to send those protons back 517 00:32:47,420 --> 00:32:50,930 through the ATP synthase, complex V, F1F0-ATPase, 518 00:32:50,930 --> 00:32:53,010 whatever you want to call it. 519 00:32:53,010 --> 00:32:55,190 And if some of those protons just 520 00:32:55,190 --> 00:32:58,160 leak back across the membrane, well, that's 521 00:32:58,160 --> 00:32:59,720 how we generate heat. 522 00:32:59,720 --> 00:33:02,480 And so how tight that membrane is, 523 00:33:02,480 --> 00:33:09,380 that is how coupled delta psi is, 524 00:33:09,380 --> 00:33:15,330 delta pH is to ATP synthesis is another variable. 525 00:33:15,330 --> 00:33:28,180 And of course, the last one is, is the efficiency of H+ turning 526 00:33:28,180 --> 00:33:33,730 the F0F1-ATPase, which is the same considerations that we 527 00:33:33,730 --> 00:33:35,230 already talked about for number two. 528 00:33:35,230 --> 00:33:39,160 That is, the higher delta psi/delta pH, the more free 529 00:33:39,160 --> 00:33:42,850 energy that's stored there and the higher or lower 530 00:33:42,850 --> 00:33:49,030 the ATP/ADP ratio is, the easier or harder it is to synthesize 531 00:33:49,030 --> 00:33:50,530 ADP. 532 00:33:50,530 --> 00:33:54,790 And so basically, these different things, 533 00:33:54,790 --> 00:34:00,040 two through four, in general takes into account 534 00:34:00,040 --> 00:34:05,290 a term called coupled respiration. 535 00:34:05,290 --> 00:34:07,720 By coupled respiration is really, 536 00:34:07,720 --> 00:34:10,870 how tightly is oxidation-- 537 00:34:10,870 --> 00:34:14,230 moving electrons on the electron transport chain to oxygen-- 538 00:34:14,230 --> 00:34:19,690 to phosphorylation-- synthesizing ATP from ADP? 539 00:34:19,690 --> 00:34:23,290 And so, how tight or how well-coupled 540 00:34:23,290 --> 00:34:28,719 is respiration to phosphorylation really 541 00:34:28,719 --> 00:34:34,360 is a variable that is going to very much depend on conditions. 542 00:34:34,360 --> 00:34:37,840 And so ultimately, to give numbers 543 00:34:37,840 --> 00:34:42,130 for what is the ATP produced from various oxidation 544 00:34:42,130 --> 00:34:44,710 processes, one has to make assumptions 545 00:34:44,710 --> 00:34:47,560 about all of these things, how coupled respiration is 546 00:34:47,560 --> 00:34:49,179 and which redox shuttles are used? 547 00:34:49,179 --> 00:34:51,219 If it's something like glycolysis, 548 00:34:51,219 --> 00:34:54,040 it involves the cytosol. 549 00:34:54,040 --> 00:34:56,170 And this is why these numbers are made up. 550 00:34:56,170 --> 00:35:00,490 And it makes no sense in my view to memorize these numbers. 551 00:35:00,490 --> 00:35:04,030 But I think it is important to understand 552 00:35:04,030 --> 00:35:06,170 where those numbers come from. 553 00:35:06,170 --> 00:35:09,160 Because if you understand that, that means you understand 554 00:35:09,160 --> 00:35:12,700 all the assumptions and why the numbers are variable, 555 00:35:12,700 --> 00:35:15,250 you really understand the bioenergetics 556 00:35:15,250 --> 00:35:17,870 of what's going on in metabolism. 557 00:35:17,870 --> 00:35:18,370 All right. 558 00:35:18,370 --> 00:35:21,760 So now let's go through one of these calculations 559 00:35:21,760 --> 00:35:25,340 just to fully illustrate what I'm talking about. 560 00:35:25,340 --> 00:35:27,930 So let's go through the calculation for glycolysis. 561 00:35:27,930 --> 00:35:34,900 So if I run glycolysis and turn glucose into two pyruvate. 562 00:35:34,900 --> 00:35:38,100 So what do we get from that? 563 00:35:38,100 --> 00:35:42,160 Of course, we get 2 ATPs produced 564 00:35:42,160 --> 00:35:48,570 and we get two NADHs produced in the cytosol. 565 00:35:48,570 --> 00:35:50,940 OK. 566 00:35:50,940 --> 00:35:54,290 So this is the cytosol. 567 00:35:54,290 --> 00:35:55,850 This is the mitochondria. 568 00:35:55,850 --> 00:35:57,300 All right? 569 00:35:57,300 --> 00:35:59,540 We're going to completely oxidize that pyruvate. 570 00:35:59,540 --> 00:36:03,140 We send that pyruvate into the mitochondria. 571 00:36:07,890 --> 00:36:10,810 We run the PDH reaction. 572 00:36:10,810 --> 00:36:14,470 That gives us two acetyl-CoA, which 573 00:36:14,470 --> 00:36:20,960 is a yield of two more NADHs. 574 00:36:20,960 --> 00:36:22,010 All right? 575 00:36:22,010 --> 00:36:27,650 If we now take those acetyl-CoAs and we burn them completely, 576 00:36:27,650 --> 00:36:31,250 two cycles around the TCA cycle. 577 00:36:31,250 --> 00:36:43,180 OK, now we get two more FADH2s and six NADHs-- 578 00:36:43,180 --> 00:36:47,710 one FADH2, three NADHs from each turn, as well as a GDP. 579 00:36:47,710 --> 00:36:54,250 And so that's a total of two FADH2, six NADHs, and two GTPs. 580 00:36:54,250 --> 00:36:59,440 And so now if I come together, what is our yield? 581 00:36:59,440 --> 00:37:03,250 Well, from the mitochondria I've generated 582 00:37:03,250 --> 00:37:10,254 a total of two FADH2s and six, seven, eight NADHs. 583 00:37:13,000 --> 00:37:18,280 And then up here, there's an additional two NADHs 584 00:37:18,280 --> 00:37:20,680 that are made in the cytosol, and so 585 00:37:20,680 --> 00:37:26,310 have to access the electron transport chain via a shuttle. 586 00:37:26,310 --> 00:37:31,890 A typical number that's given is you get 1.58 ATP 587 00:37:31,890 --> 00:37:35,250 for something that has to access via shuttle, 588 00:37:35,250 --> 00:37:40,770 or on FADH2, that would make sense, 589 00:37:40,770 --> 00:37:44,370 because FADH2 is really the same as a glycerol phosphate 590 00:37:44,370 --> 00:37:45,240 shuttle. 591 00:37:45,240 --> 00:37:50,580 And so if I do that, that would be three ATP from the two 592 00:37:50,580 --> 00:38:00,720 glycolysis NADHs and three ATP from the two TCA cycle FADH2s, 593 00:38:00,720 --> 00:38:05,940 because the NADHs produced in the matrix can generate 594 00:38:05,940 --> 00:38:09,930 POM protons via complex I. Oftentimes, that's said to be, 595 00:38:09,930 --> 00:38:12,150 oh, that's about 2 and 1/2 ATP. 596 00:38:12,150 --> 00:38:16,060 So 8 times 2.5 is 20 ATP. 597 00:38:16,060 --> 00:38:17,790 And if we had all these numbers together, 598 00:38:17,790 --> 00:38:27,210 20 plus 3 plus 3 plus 2 plus 2, that is a total of 30 599 00:38:27,210 --> 00:38:35,040 ATP per glucose that's completely oxidized to CO2. 600 00:38:35,040 --> 00:38:36,180 OK? 601 00:38:36,180 --> 00:38:46,010 So 26 ATP over here plus another 2 from GTP plus another 2 602 00:38:46,010 --> 00:38:50,570 from glycolysis is a total of 20 ATP per glucose. 603 00:38:50,570 --> 00:38:51,470 That is a number. 604 00:38:51,470 --> 00:38:53,000 It made certain assumptions. 605 00:38:53,000 --> 00:38:58,400 Obviously, if I assume that this is not the glycerol phosphate 606 00:38:58,400 --> 00:39:00,800 shuttle but instead the malate aspartate shuttle, 607 00:39:00,800 --> 00:39:05,150 maybe I bump this number up by multiplying that by 2.5. 608 00:39:05,150 --> 00:39:08,240 Maybe I use different conversion factors. 609 00:39:08,240 --> 00:39:10,610 And if you read in textbooks, you 610 00:39:10,610 --> 00:39:14,090 get numbers that range from 30 to 36. 611 00:39:14,090 --> 00:39:19,490 I think the highest I've ever seen is 38 ATP per glucose. 612 00:39:19,490 --> 00:39:22,370 And the fact that you actually see this range really 613 00:39:22,370 --> 00:39:26,030 reflects the different authors of textbooks, 614 00:39:26,030 --> 00:39:29,120 or whoever writes these things, makes different assumptions 615 00:39:29,120 --> 00:39:32,630 about all of these things over here 616 00:39:32,630 --> 00:39:35,180 to understand how to make these conversions. 617 00:39:35,180 --> 00:39:37,580 But the important thing is realizing that these are not 618 00:39:37,580 --> 00:39:39,220 real numbers. 619 00:39:39,220 --> 00:39:39,720 All right. 620 00:39:39,720 --> 00:39:42,150 Let's quickly go through and just do 621 00:39:42,150 --> 00:39:45,090 the similar exercise one last time just to come back to. 622 00:39:45,090 --> 00:39:47,610 I'm not going to go through as much detail. 623 00:39:47,610 --> 00:39:50,640 But we talked about that 6:0 fatty acid 624 00:39:50,640 --> 00:39:52,650 that we did accounting for earlier. 625 00:39:52,650 --> 00:39:55,440 If you look back in your notes, what we wrote down 626 00:39:55,440 --> 00:40:00,210 is that from completely oxidizing that fully saturated 627 00:40:00,210 --> 00:40:09,970 six carbon fatty acid, we got one ATP, 11 and ADHDs, and five 628 00:40:09,970 --> 00:40:10,470 FADH2s. 629 00:40:13,960 --> 00:40:16,260 Remember, all these are being generated 630 00:40:16,260 --> 00:40:20,850 in the-- all these NADHs and FADH2s are already 631 00:40:20,850 --> 00:40:22,320 in the mitochondria. 632 00:40:22,320 --> 00:40:26,070 And so if we just use the same conversion factors 633 00:40:26,070 --> 00:40:31,170 that we used for over there in our calculation for glycolysis, 634 00:40:31,170 --> 00:40:44,280 what we get is 27.5 ATP for the NADHs and 7.5 ATP 635 00:40:44,280 --> 00:40:54,640 for the FADH2s, which is a total of 35 ATP for-- 636 00:40:58,130 --> 00:41:05,320 35 plus the one, so a total of 36 637 00:41:05,320 --> 00:41:11,327 ATP from complete oxidation of our six carbon fatty acid, 638 00:41:11,327 --> 00:41:12,910 which is certainly greater than the 30 639 00:41:12,910 --> 00:41:17,200 ATP we got from complete oxidation of glucose. 640 00:41:17,200 --> 00:41:18,730 That makes sense. 641 00:41:18,730 --> 00:41:21,460 Fatty acids are more reduced than glucose-- 642 00:41:21,460 --> 00:41:24,640 more calories, should be able to get more from it. 643 00:41:24,640 --> 00:41:28,470 And this calculation illustrates that we get that. 644 00:41:28,470 --> 00:41:31,110 The typical calculation in textbook 645 00:41:31,110 --> 00:41:35,730 usually describes complete oxidation of a 16:0-- 646 00:41:35,730 --> 00:41:39,600 a fully saturated 16 carbon fatty acid. 647 00:41:39,600 --> 00:41:40,530 That's palmitate. 648 00:41:40,530 --> 00:41:43,830 That's one of the most abundant fatty acids in the cell. 649 00:41:43,830 --> 00:41:46,350 This, if you completely oxidize it, 650 00:41:46,350 --> 00:41:48,420 the number and a lot of textbooks 651 00:41:48,420 --> 00:41:57,540 is 106 ATP per palmitate completely oxidized to CO2. 652 00:41:57,540 --> 00:41:59,790 If you're so inclined, you can go through 653 00:41:59,790 --> 00:42:04,150 and do the calculations yourself and see if you agree with that. 654 00:42:04,150 --> 00:42:07,350 But the key point of all of this accounting is that none of this 655 00:42:07,350 --> 00:42:08,980 are real numbers. 656 00:42:08,980 --> 00:42:11,850 And I hope you now better appreciate 657 00:42:11,850 --> 00:42:14,730 what energy really means-- 658 00:42:14,730 --> 00:42:19,440 that is, how oxidation of carbon by being favorable 659 00:42:19,440 --> 00:42:26,220 can be coupled either to direct charging of an ATP/ADT ratio 660 00:42:26,220 --> 00:42:31,440 as occurs in the GAPDH step of glycolysis, 661 00:42:31,440 --> 00:42:34,380 as well as the succinic thiokinase 662 00:42:34,380 --> 00:42:36,330 step of the TCA cycle. 663 00:42:36,330 --> 00:42:39,510 But most energy transduction involves 664 00:42:39,510 --> 00:42:43,800 this charging up NAD/NADH ratios, 665 00:42:43,800 --> 00:42:47,670 or reactions that are coupled to FAD/FADH2 666 00:42:47,670 --> 00:42:52,050 via these membrane complexes that ultimately lead 667 00:42:52,050 --> 00:42:54,930 to this favorable electron transport 668 00:42:54,930 --> 00:42:59,700 from lower to higher standard reduction potentials. 669 00:42:59,700 --> 00:43:02,160 That favorable electron transport 670 00:43:02,160 --> 00:43:06,030 can be used to create a battery, a delta psi/delta pH, which 671 00:43:06,030 --> 00:43:08,470 can then be coupled to do other work, 672 00:43:08,470 --> 00:43:13,170 including synthesizing ATP at the high ATP/ADP ratio that 673 00:43:13,170 --> 00:43:15,630 can then be used by other reactions 674 00:43:15,630 --> 00:43:20,100 in the cell that would otherwise be unfavorable and allow cells 675 00:43:20,100 --> 00:43:23,790 to basically extract energy from its environment 676 00:43:23,790 --> 00:43:28,530 in order to fuel the processes that are unfavorable and are 677 00:43:28,530 --> 00:43:31,860 necessary to sustain life. 678 00:43:31,860 --> 00:43:33,785 All right. 679 00:43:33,785 --> 00:43:34,285 Great. 680 00:43:40,510 --> 00:43:47,270 So thus far, we have focused almost entirely 681 00:43:47,270 --> 00:43:54,160 on how you oxidized carbon as a way to release energy. 682 00:43:54,160 --> 00:43:55,590 And that's great. 683 00:43:55,590 --> 00:43:57,880 But of course, that reduced carbon 684 00:43:57,880 --> 00:44:01,300 has to come from somewhere to begin with. 685 00:44:01,300 --> 00:44:04,870 And every one of us learned in grade school 686 00:44:04,870 --> 00:44:09,220 that the energy for all life ultimately comes from the sun. 687 00:44:09,220 --> 00:44:11,080 And that's, of course, photosynthesis. 688 00:44:11,080 --> 00:44:13,600 And now I want to turn to beginning 689 00:44:13,600 --> 00:44:18,110 to discuss photosynthesis and how it works. 690 00:44:18,110 --> 00:44:21,880 In other words, how does life harvest solar energy 691 00:44:21,880 --> 00:44:31,550 in a usable form that ultimately involves using atmospheric CO2, 692 00:44:31,550 --> 00:44:35,540 which gets reduced to generate reduced 693 00:44:35,540 --> 00:44:38,060 carbon that, of course, originally 694 00:44:38,060 --> 00:44:40,790 occurred because plants-- 695 00:44:40,790 --> 00:44:44,990 or, any photosynthetic organism has the same issues as us. 696 00:44:44,990 --> 00:44:48,650 That is, to fight thermodynamics, maintain, 697 00:44:48,650 --> 00:44:49,730 and survive. 698 00:44:49,730 --> 00:44:54,410 It constantly has to be charging an ATP/ADP ratio in order 699 00:44:54,410 --> 00:44:58,070 to fuel the otherwise unfavorable processes that 700 00:44:58,070 --> 00:45:02,600 are required to maintain order in the cell. 701 00:45:02,600 --> 00:45:05,450 This has to occur during the day when the sun is out 702 00:45:05,450 --> 00:45:08,150 and there's light energy photosynthesis. 703 00:45:08,150 --> 00:45:10,820 But also has to occur at night, or at times 704 00:45:10,820 --> 00:45:12,320 when there is no sun. 705 00:45:12,320 --> 00:45:19,070 And so thus, plants basically stored energy as reduced carbon 706 00:45:19,070 --> 00:45:21,770 so that they had something to eat at night-- that is, 707 00:45:21,770 --> 00:45:24,620 to keep their ATP/ADP ratios high-- 708 00:45:24,620 --> 00:45:27,830 at night so they can do all these catabolic processes 709 00:45:27,830 --> 00:45:29,210 at night. 710 00:45:29,210 --> 00:45:32,540 Now, fortunately for us, the fact 711 00:45:32,540 --> 00:45:36,590 that photosynthetic organisms stored all this reduced carbon 712 00:45:36,590 --> 00:45:39,950 allowed other life, like us, to evolve 713 00:45:39,950 --> 00:45:45,200 that is entirely dependent on photosynthetic organisms 714 00:45:45,200 --> 00:45:49,380 either directly or indirectly for food. 715 00:45:49,380 --> 00:45:55,160 And so photosynthesis also is the process that ultimately 716 00:45:55,160 --> 00:45:59,670 oxygenated our atmosphere. 717 00:45:59,670 --> 00:46:03,120 Aerobic life, also something that was necessary for us 718 00:46:03,120 --> 00:46:07,770 to survive because oxygen is such a key electron acceptor, 719 00:46:07,770 --> 00:46:10,750 really key to our energetics-- 720 00:46:10,750 --> 00:46:13,140 well, the fact that photosynthetic organisms 721 00:46:13,140 --> 00:46:16,200 put oxygen in our atmosphere also 722 00:46:16,200 --> 00:46:23,370 was really essential for aerobic life, us, to ultimately evolve. 723 00:46:23,370 --> 00:46:28,530 And so photosynthesis is often, in biochemistry courses, almost 724 00:46:28,530 --> 00:46:31,660 a sidelight and forgotten process. 725 00:46:31,660 --> 00:46:34,410 But of course, it is really, really central 726 00:46:34,410 --> 00:46:36,840 to how life works, because no life 727 00:46:36,840 --> 00:46:38,940 could exist without photosynthesis 728 00:46:38,940 --> 00:46:41,700 and the conditions that allowed aerobic-- 729 00:46:41,700 --> 00:46:43,710 most multicellular life that we know 730 00:46:43,710 --> 00:46:46,890 of that's not photosynthetic, including ourselves, 731 00:46:46,890 --> 00:46:50,130 could not have evolved without photosynthesis 732 00:46:50,130 --> 00:46:52,770 oxygenating the atmosphere and generating 733 00:46:52,770 --> 00:46:54,390 all this reduced carbon. 734 00:46:54,390 --> 00:46:59,370 That, really, is where we get our energy from as food. 735 00:46:59,370 --> 00:47:02,760 Now, you also learned in grade school, 736 00:47:02,760 --> 00:47:06,330 when you learned that life depends on energy of the sun, 737 00:47:06,330 --> 00:47:09,390 that the process that life uses is 738 00:47:09,390 --> 00:47:13,080 this mysterious photosynthesis. 739 00:47:13,080 --> 00:47:15,360 Well, as we go through this over the rest of today 740 00:47:15,360 --> 00:47:17,790 and in the next lecture, I want you to appreciate 741 00:47:17,790 --> 00:47:20,220 what photosynthesis is. 742 00:47:20,220 --> 00:47:26,130 And that is how life really takes solar energy 743 00:47:26,130 --> 00:47:30,990 and harvests that energy in a biologically useful form, 744 00:47:30,990 --> 00:47:35,310 bearing in mind all of the concepts that we've already 745 00:47:35,310 --> 00:47:41,110 talked about, because that will ultimately help you understand 746 00:47:41,110 --> 00:47:45,730 how that energy from the sun is used to make all 747 00:47:45,730 --> 00:47:49,180 this reduced carbon, this carbohydrate and fat 748 00:47:49,180 --> 00:47:53,420 that is energy that we can use for later, and also, of course, 749 00:47:53,420 --> 00:47:57,220 allows how photosynthetic organisms can use light 750 00:47:57,220 --> 00:47:59,680 from the sun to just power things directly 751 00:47:59,680 --> 00:48:04,090 from that solar battery, and ultimately, of course, 752 00:48:04,090 --> 00:48:09,030 you should appreciate that these things all came first. 753 00:48:09,030 --> 00:48:11,730 Photosynthesis obviously existed first. 754 00:48:11,730 --> 00:48:14,520 And so most of what we already talked about-- 755 00:48:14,520 --> 00:48:17,700 well, we'll draw parallels when we talk about photosynthesis 756 00:48:17,700 --> 00:48:19,320 to the stuff we already know. 757 00:48:19,320 --> 00:48:21,290 Of course, photosynthesis came first. 758 00:48:21,290 --> 00:48:23,040 And the stuff you've already learned about 759 00:48:23,040 --> 00:48:26,490 evolved from photosynthesis. 760 00:48:26,490 --> 00:48:27,600 All right. 761 00:48:27,600 --> 00:48:31,770 Now, like all pathways, photosynthesis also 762 00:48:31,770 --> 00:48:35,670 has to follow the same rules and laws of thermodynamics that 763 00:48:35,670 --> 00:48:37,530 were true for all pathways. 764 00:48:37,530 --> 00:48:41,890 So plants and animals have to follow the same rules. 765 00:48:41,890 --> 00:48:44,220 And so what is photosynthesis? 766 00:48:44,220 --> 00:48:48,840 Well, it's ultimately CO2 in the atmosphere that 767 00:48:48,840 --> 00:48:53,100 gets reduced to carbohydrate. 768 00:48:56,500 --> 00:48:58,240 That's classically photosynthesis. 769 00:48:58,240 --> 00:49:03,070 But, of course, plants also make fatty acids. 770 00:49:03,070 --> 00:49:04,450 OK? 771 00:49:04,450 --> 00:49:08,830 This is carbon reduction. 772 00:49:08,830 --> 00:49:13,090 Remember, carbon oxidation generally is favorable. 773 00:49:13,090 --> 00:49:15,340 Carbon reduction is not. 774 00:49:15,340 --> 00:49:20,830 Delta G naught prime for this is greater than 0. 775 00:49:20,830 --> 00:49:22,510 This direction is favored. 776 00:49:22,510 --> 00:49:24,760 That direction is not, which means 777 00:49:24,760 --> 00:49:28,750 you need energy input to reduce that CO2 778 00:49:28,750 --> 00:49:30,940 to carbohydrate or fatty acid. 779 00:49:30,940 --> 00:49:35,290 And of course, it's the sun's energy, the solar energy, 780 00:49:35,290 --> 00:49:40,530 that ultimately must be done to make this favorable. 781 00:49:40,530 --> 00:49:42,960 Now we're reducing carbon. 782 00:49:42,960 --> 00:49:45,750 That means we need to add electrons. 783 00:49:45,750 --> 00:49:48,430 Those electrons have to come from somewhere. 784 00:49:48,430 --> 00:49:51,750 And so we're reducing the carbon. 785 00:49:51,750 --> 00:49:54,210 That means something else has to be oxidized. 786 00:49:54,210 --> 00:49:56,550 Just like when we oxidized carbon, 787 00:49:56,550 --> 00:49:58,170 those electrons had to go somewhere, 788 00:49:58,170 --> 00:50:00,300 something else had to be reduced. 789 00:50:00,300 --> 00:50:02,910 That was pyruvate to lactate and fermentation, 790 00:50:02,910 --> 00:50:08,070 or oxygen to water via all of the oxidative phosphorylation 791 00:50:08,070 --> 00:50:09,690 things we just talked about. 792 00:50:09,690 --> 00:50:11,190 And so in this case, those electrons 793 00:50:11,190 --> 00:50:12,700 have to come from somewhere. 794 00:50:12,700 --> 00:50:17,340 And those electrons ultimately, for photosynthesis-- 795 00:50:17,340 --> 00:50:19,650 this reduction, the electrons come 796 00:50:19,650 --> 00:50:26,420 from oxidation, ultimately, of water, 797 00:50:26,420 --> 00:50:34,020 which will generate oxygen. And this is effectively then 798 00:50:34,020 --> 00:50:36,390 the reverse of what you learned about 799 00:50:36,390 --> 00:50:39,840 for the mitochondrial electron transport. 800 00:50:39,840 --> 00:50:40,890 Why this? 801 00:50:40,890 --> 00:50:49,980 Well, water is very abundant, has been for all of life 802 00:50:49,980 --> 00:50:50,700 on Earth. 803 00:50:50,700 --> 00:50:55,480 Water is-- obviously, life depends on water. 804 00:50:55,480 --> 00:51:02,820 And so you can oxidize the water to generate 805 00:51:02,820 --> 00:51:06,030 oxygen. This will obviously generate electrons that 806 00:51:06,030 --> 00:51:09,390 can be used to reduce carbon. 807 00:51:09,390 --> 00:51:13,140 And it's effectively the photosynthetic oxidation 808 00:51:13,140 --> 00:51:17,970 of water to make oxygen that was the source of oxygen 809 00:51:17,970 --> 00:51:21,030 in the atmosphere that, really, today 810 00:51:21,030 --> 00:51:24,040 makes aerobic life possible. 811 00:51:24,040 --> 00:51:27,358 Now, of course, the fact that oxygen is a good electron 812 00:51:27,358 --> 00:51:29,400 acceptor-- we've been talking about this forever. 813 00:51:29,400 --> 00:51:31,710 This means that, of course, you might 814 00:51:31,710 --> 00:51:36,180 guess that delta G naught prime for this process 815 00:51:36,180 --> 00:51:39,660 is not going to be favorable. 816 00:51:39,660 --> 00:51:42,690 To put it another way, oxygen is a great electron 817 00:51:42,690 --> 00:51:46,360 acceptor and water is a poor electron donor. 818 00:51:46,360 --> 00:51:49,350 And so, really, the way photosynthesis-- 819 00:51:49,350 --> 00:51:51,780 the magic of photosynthesis, if you will, 820 00:51:51,780 --> 00:52:00,300 is making water a good electron donor, because, ultimately, 821 00:52:00,300 --> 00:52:03,840 that is what is going to make this whole thing 822 00:52:03,840 --> 00:52:06,330 work energetically. 823 00:52:06,330 --> 00:52:13,620 And so you probably could guess that if these processes are 824 00:52:13,620 --> 00:52:15,990 really about these electron transfers, 825 00:52:15,990 --> 00:52:17,760 we need electron carriers. 826 00:52:17,760 --> 00:52:20,460 This is true for photosynthesis, just 827 00:52:20,460 --> 00:52:25,080 as it was true for oxidative phosphorylation. 828 00:52:25,080 --> 00:52:27,660 But remember, we'll describe a lot of the same electron 829 00:52:27,660 --> 00:52:28,830 carriers that are used. 830 00:52:28,830 --> 00:52:30,510 But as we described them, remember 831 00:52:30,510 --> 00:52:32,410 that the photosynthesis came first. 832 00:52:32,410 --> 00:52:36,540 And so the other processes, like OXPHOS, that use these electron 833 00:52:36,540 --> 00:52:38,490 carriers, use them because they were first 834 00:52:38,490 --> 00:52:39,990 used in photosynthesis. 835 00:52:39,990 --> 00:52:45,610 And then they were repurposed for oxidative phosphorylation. 836 00:52:45,610 --> 00:52:48,370 Now, to discuss this, I want to first start 837 00:52:48,370 --> 00:52:52,540 by introducing an electron carrier that's 838 00:52:52,540 --> 00:52:57,370 important for this that's really a variation on NADH. 839 00:52:57,370 --> 00:53:00,520 And it's an electron carrier that's found in-- 840 00:53:00,520 --> 00:53:05,050 excuse me-- all plants, all animals, all life, 841 00:53:05,050 --> 00:53:08,440 and is very important in anabolic pathways 842 00:53:08,440 --> 00:53:10,960 like photosynthesis. 843 00:53:10,960 --> 00:53:13,840 Remember that anabolism, building stuff, 844 00:53:13,840 --> 00:53:17,020 has to be distinct from catabolism, breaking stuff 845 00:53:17,020 --> 00:53:18,820 down, for a number of reasons. 846 00:53:18,820 --> 00:53:21,910 We saw this in glycolysis and gluconeogenesis, glycogen 847 00:53:21,910 --> 00:53:24,430 synthesis, glycogen breakdown. 848 00:53:24,430 --> 00:53:28,160 Remember, delta G naught prime can only favor one direction. 849 00:53:28,160 --> 00:53:31,117 And so in the direction that's favorable, it works. 850 00:53:31,117 --> 00:53:33,200 In the other direction to actually make it happen, 851 00:53:33,200 --> 00:53:35,770 we have to make delta G less than zero. 852 00:53:35,770 --> 00:53:37,660 That has to happen by energy addition. 853 00:53:37,660 --> 00:53:40,900 And so now we had to have these separate pathways. 854 00:53:40,900 --> 00:53:42,820 You also need separate pathways because that 855 00:53:42,820 --> 00:53:46,270 allows separate regulation, avoids futile cycle, 856 00:53:46,270 --> 00:53:49,840 and enables cells to match needs with processes. 857 00:53:49,840 --> 00:53:52,450 And as we already discussed, one way 858 00:53:52,450 --> 00:53:55,660 to keep these anabolic and catabolic processes distinct 859 00:53:55,660 --> 00:53:57,730 is by building separate compartments, right? 860 00:53:57,730 --> 00:53:59,863 It's useful to have separate compartments, 861 00:53:59,863 --> 00:54:01,780 different conditions in different compartments 862 00:54:01,780 --> 00:54:04,820 that can favor different things happening. 863 00:54:04,820 --> 00:54:07,630 However, not all cells have separate compartments. 864 00:54:07,630 --> 00:54:09,130 Prokaryotes don't. 865 00:54:09,130 --> 00:54:11,830 And you might think of situations 866 00:54:11,830 --> 00:54:14,440 where you clearly want to have two 867 00:54:14,440 --> 00:54:16,270 different types of reactions happening 868 00:54:16,270 --> 00:54:17,680 in the same compartment. 869 00:54:17,680 --> 00:54:22,120 And so what if you want to both do oxidation and reduction 870 00:54:22,120 --> 00:54:25,230 in different reactions in the same compartment? 871 00:54:25,230 --> 00:54:28,630 Well, clearly you need some way that 872 00:54:28,630 --> 00:54:31,780 will favor some reactions being oxidation 873 00:54:31,780 --> 00:54:34,600 and other reactions being reductions. 874 00:54:34,600 --> 00:54:42,940 And if you simply rely on NADH/NAD+ as your oxidant 875 00:54:42,940 --> 00:54:47,490 or your reductant, this now becomes a problem. 876 00:54:47,490 --> 00:54:48,940 All right? 877 00:54:48,940 --> 00:54:53,590 So a solution is, well, let's make a distinct electron 878 00:54:53,590 --> 00:54:57,410 carrier for the oxidation reactions in the cells. 879 00:54:57,410 --> 00:55:00,130 So that's what this is. 880 00:55:00,130 --> 00:55:02,740 Turns out, you keep your NAD/NADH 881 00:55:02,740 --> 00:55:06,940 to largely favor the oxidation reactions you want to do. 882 00:55:06,940 --> 00:55:10,570 And let's create a different currency for reduction. 883 00:55:10,570 --> 00:55:14,326 And this currency ends up being a different molecule, 884 00:55:14,326 --> 00:55:15,300 NADPH/NADP+. 885 00:55:18,220 --> 00:55:24,415 And you use this as a cofactor pair for reduction. 886 00:55:24,415 --> 00:55:26,380 Well, what is NADPH? 887 00:55:29,510 --> 00:55:34,130 Well, it's effectively the same thing as NADH. 888 00:55:34,130 --> 00:55:36,770 But now you mark it as a separate pool 889 00:55:36,770 --> 00:55:42,290 with a two prime phosphate on the ADP portion 890 00:55:42,290 --> 00:55:43,765 of the molecule. 891 00:55:43,765 --> 00:55:44,890 Let me draw it out for you. 892 00:56:26,470 --> 00:56:27,010 OK. 893 00:56:27,010 --> 00:56:30,400 So this here is NA-- 894 00:56:30,400 --> 00:56:34,600 so if I took this phosphate off, this would be NADH, 895 00:56:34,600 --> 00:56:37,480 put a two prime phosphate on, now I 896 00:56:37,480 --> 00:56:41,500 mark this as a separate molecule pool, NADPH. 897 00:56:49,409 --> 00:56:50,600 A is an adenine. 898 00:56:50,600 --> 00:56:54,630 Remember, it's a nicotinamide adenine dinucleoside. 899 00:56:54,630 --> 00:56:55,130 OK? 900 00:56:55,130 --> 00:56:59,510 So here's a adenine, phosphate, phosphate, other nucleoside 901 00:56:59,510 --> 00:57:01,100 with the nicotinamide group. 902 00:57:01,100 --> 00:57:03,890 This here is in the reduced form. 903 00:57:07,480 --> 00:57:12,000 Seen this now a million times, but we 904 00:57:12,000 --> 00:57:14,760 can oxidize it as possible. 905 00:57:14,760 --> 00:57:19,280 That generates this two electron hydride ion. 906 00:57:19,280 --> 00:57:35,500 So remove two electrons and now this nicotinamide form is here 907 00:57:35,500 --> 00:57:45,860 in the NADP+ or oxidized form of the molecule. 908 00:57:45,860 --> 00:57:49,300 And so effectively by marking this as a separate pool with 909 00:57:49,300 --> 00:57:55,300 that two prime phosphate, you can have a different NADH/NAD+ 910 00:57:55,300 --> 00:57:56,740 ratio in cells. 911 00:57:56,740 --> 00:58:01,870 Then you have NADH/NADP+ plus ratio and cells. 912 00:58:01,870 --> 00:58:04,510 Use one co-factor fair pair to favor 913 00:58:04,510 --> 00:58:07,390 oxidation and the other co-factor pair 914 00:58:07,390 --> 00:58:10,490 to favor reduction. 915 00:58:10,490 --> 00:58:14,480 And so I want to give a quick aside on NADPH 916 00:58:14,480 --> 00:58:17,510 because it's really also NADPH that 917 00:58:17,510 --> 00:58:22,640 can be very important in keeping the cytosol of cells reduced. 918 00:58:22,640 --> 00:58:23,900 So what do I mean by that? 919 00:58:23,900 --> 00:58:27,370 Well, you remember you learned from Professor Yaffe. 920 00:58:27,370 --> 00:58:30,530 We talked about proteins that you can have 921 00:58:30,530 --> 00:58:32,900 these inside the side cytosol. 922 00:58:32,900 --> 00:58:36,170 You'd have your cystine residues on proteins 923 00:58:36,170 --> 00:58:38,480 be in this reduced form. 924 00:58:38,480 --> 00:58:42,950 And outside the cell, you'd form these disulfide bonds, which 925 00:58:42,950 --> 00:58:45,440 is basically oxidized cystine. 926 00:58:45,440 --> 00:58:50,150 And so in the cytosol, you want your cystine residues reduced. 927 00:58:50,150 --> 00:58:52,100 And outside of the cell, you end up 928 00:58:52,100 --> 00:58:56,380 with these disulfide bonds on proteins. 929 00:58:56,380 --> 00:59:01,330 Why does cystine tend to get oxidized outside the cell? 930 00:59:01,330 --> 00:59:04,420 Well, there's lots of oxygen in our atmosphere. 931 00:59:04,420 --> 00:59:06,580 Oxygen's a good electron acceptor. 932 00:59:06,580 --> 00:59:08,650 It's very good at oxidation. 933 00:59:08,650 --> 00:59:12,100 In fact, that's what gives it this property that makes 934 00:59:12,100 --> 00:59:14,410 oxidative phosphorylation work. 935 00:59:14,410 --> 00:59:17,530 And so it turns out that these cystines, 936 00:59:17,530 --> 00:59:20,890 these sulfhydryl groups, like to give their electrons to oxygen. 937 00:59:20,890 --> 00:59:24,100 And that leads to these more oxidized things 938 00:59:24,100 --> 00:59:25,760 outside the cell. 939 00:59:25,760 --> 00:59:28,960 In fact, oxygen is very good at accepting electrons 940 00:59:28,960 --> 00:59:30,940 from lots of donors. 941 00:59:30,940 --> 00:59:35,740 So if you have iron 2+ sitting around outside the cell, 942 00:59:35,740 --> 00:59:41,950 oxygen is very good at oxidizing that iron 2+ to iron 3+. 943 00:59:41,950 --> 00:59:43,840 That's rust. 944 00:59:43,840 --> 00:59:48,130 And this process, in the end, ends up transferring electrons 945 00:59:48,130 --> 00:59:52,850 to oxygen and can generate lots of oxygen species, 946 00:59:52,850 --> 00:59:54,830 including oxygen radicals. 947 00:59:54,830 --> 01:00:00,000 And so you can get things like this O2- superoxide, 948 01:00:00,000 --> 01:00:11,170 ultimately hydrogen peroxide, also 949 01:00:11,170 --> 01:00:15,380 comes from oxygen picking up electrons. 950 01:00:15,380 --> 01:00:19,090 And as you know, very reactive molecules like this 951 01:00:19,090 --> 01:00:23,380 can damage membranes, damage proteins, kill cells. 952 01:00:23,380 --> 01:00:25,930 What do we use hydrogen peroxide for in daily life? 953 01:00:25,930 --> 01:00:29,080 Use it to clean wounds, kills a bunch of bacteria. 954 01:00:29,080 --> 01:00:34,900 And ultimately, production of these so-called ROS, 955 01:00:34,900 --> 01:00:42,260 or reactive oxygen species, in excess 956 01:00:42,260 --> 01:00:45,170 can be very bad for biological systems. 957 01:00:45,170 --> 01:00:47,300 And this is one of the arguments for why 958 01:00:47,300 --> 01:00:48,860 antioxidants are good for us. 959 01:00:48,860 --> 01:00:53,900 They protect us and ourselves from these reactive oxygen 960 01:00:53,900 --> 01:00:55,940 species. 961 01:00:55,940 --> 01:00:56,960 OK. 962 01:00:56,960 --> 01:01:00,260 Now, cells, of course, have endogenous ways 963 01:01:00,260 --> 01:01:03,560 to deal with this, to keep cystines in our cytosol 964 01:01:03,560 --> 01:01:06,440 reduced, to keep our cytosol in this reducing environment, 965 01:01:06,440 --> 01:01:09,200 prevent this ROS damage. 966 01:01:09,200 --> 01:01:11,480 And really, if you're going to have 967 01:01:11,480 --> 01:01:13,760 things to deal with these reactive molecules, what do 968 01:01:13,760 --> 01:01:14,180 you want? 969 01:01:14,180 --> 01:01:15,722 Well, you want something that's going 970 01:01:15,722 --> 01:01:19,963 to buffer the damage that these things can cause. 971 01:01:19,963 --> 01:01:21,380 That is, if these things are going 972 01:01:21,380 --> 01:01:24,800 to react with lipids and proteins and cause damage 973 01:01:24,800 --> 01:01:26,510 and you want to keep the cytosol reduced, 974 01:01:26,510 --> 01:01:28,520 you really want to have a series of molecules 975 01:01:28,520 --> 01:01:33,050 that will actually, instead, undergo this process before it 976 01:01:33,050 --> 01:01:35,840 can damage critical structures in the cell. 977 01:01:35,840 --> 01:01:38,270 And a key system cells use to do this is 978 01:01:38,270 --> 01:01:42,020 something called glutathione. 979 01:01:42,020 --> 01:01:44,120 So what is glutathione? 980 01:01:44,120 --> 01:01:48,480 Well, glutathione is a small molecule tripeptide. 981 01:01:48,480 --> 01:01:48,980 All right? 982 01:01:48,980 --> 01:01:54,320 And it's among the most abundant small molecules found in cells. 983 01:01:54,320 --> 01:01:56,480 And effectively, what glutathione does 984 01:01:56,480 --> 01:02:00,680 is it serves this purpose of reacting with nasty stuff 985 01:02:00,680 --> 01:02:06,530 before it can damage other biomolecules and hurt the cell. 986 01:02:06,530 --> 01:02:09,440 And so by tripeptide, what it is is it's 987 01:02:09,440 --> 01:02:14,810 a tripeptide of glutamate with a gamma peptide bond 988 01:02:14,810 --> 01:02:16,790 to glycine and then cystine-- 989 01:02:16,790 --> 01:02:20,900 so glutamate glycine cystine tripeptide 990 01:02:20,900 --> 01:02:22,190 that looks like this. 991 01:02:38,170 --> 01:02:38,670 OK. 992 01:02:38,670 --> 01:02:43,520 So this here is the amino acid glutamate. 993 01:02:43,520 --> 01:02:48,602 Here's the alpha carbon with the carboxylic acid 994 01:02:48,602 --> 01:02:49,310 and amine groups. 995 01:02:49,310 --> 01:02:52,490 So you would typically think of the peptide bond of glutamate 996 01:02:52,490 --> 01:02:55,910 was in a chain as being from this nitrogen 997 01:02:55,910 --> 01:02:59,480 to the next amino acid and that carbon to the next amino acid. 998 01:02:59,480 --> 01:03:03,210 But in this case, it uses the nitrogen on the side chain, 999 01:03:03,210 --> 01:03:06,560 the so-called gamma linkage here, 1000 01:03:06,560 --> 01:03:09,380 to form a peptide bond with-- 1001 01:03:12,430 --> 01:03:15,400 oops, sorry. 1002 01:03:15,400 --> 01:03:17,620 This would be glutamic acid. 1003 01:03:17,620 --> 01:03:20,410 It uses this gamma carboxylic acid 1004 01:03:20,410 --> 01:03:26,790 on the side chain of glutamate to form a peptide bond 1005 01:03:26,790 --> 01:03:29,790 with glycine. 1006 01:03:33,220 --> 01:03:34,180 OK. 1007 01:03:34,180 --> 01:03:39,210 So this is glutamate. 1008 01:03:39,210 --> 01:03:41,730 This is glycine. 1009 01:03:41,730 --> 01:03:59,740 And then this has a peptide bond to cystine, 1010 01:03:59,740 --> 01:04:03,610 which contributes that sulfhydryl group that 1011 01:04:03,610 --> 01:04:07,720 can either exist in this reduced form. 1012 01:04:07,720 --> 01:04:14,780 Or, if there's two of them together, 1013 01:04:14,780 --> 01:04:18,340 you can go into this oxidized form. 1014 01:04:18,340 --> 01:04:25,330 And so, often, this is drawn as glutathione GSH in the reduced 1015 01:04:25,330 --> 01:04:26,450 form. 1016 01:04:26,450 --> 01:04:32,260 And if you have two glutathione molecules that then basically 1017 01:04:32,260 --> 01:04:37,150 form a disulfide bond to give you this GSSG 1018 01:04:37,150 --> 01:04:41,050 oxidized form of the molecule. 1019 01:04:41,050 --> 01:04:45,430 And NADPH plays a key role in the cell, 1020 01:04:45,430 --> 01:04:46,990 because that's an oxidation reduction 1021 01:04:46,990 --> 01:04:50,800 reaction in keeping glutathione in the reduced form. 1022 01:04:50,800 --> 01:04:55,480 So if you have two GSH molecules that 1023 01:04:55,480 --> 01:05:00,070 can cycle to a oxidized GSSG-- 1024 01:05:00,070 --> 01:05:06,230 so this is reduced, this is oxidized. 1025 01:05:06,230 --> 01:05:09,880 On the other side you can cycle those electrons. 1026 01:05:09,880 --> 01:05:11,630 If something gets oxidized, something else 1027 01:05:11,630 --> 01:05:13,260 has to be reduced. 1028 01:05:13,260 --> 01:05:18,410 So you can have NADP+ and NADPH. 1029 01:05:23,650 --> 01:05:26,660 This is reduced. 1030 01:05:26,660 --> 01:05:28,100 This is oxidized. 1031 01:05:28,100 --> 01:05:34,640 And so by maintaining a high ratio of NADH to NADP+, 1032 01:05:34,640 --> 01:05:39,590 you can keep a high ratio of reduced to oxidize glutathione 1033 01:05:39,590 --> 01:05:44,030 in the cell and keep the cell in a protected reducing 1034 01:05:44,030 --> 01:05:49,220 environment where you have these glutathione molecules to react 1035 01:05:49,220 --> 01:05:53,630 with other things before it causes damage to the cell. 1036 01:05:53,630 --> 01:05:54,510 All right. 1037 01:05:54,510 --> 01:05:58,250 A little bit of a diversion, but a very important system 1038 01:05:58,250 --> 01:06:02,720 that cells use related to oxidation reduction and NADPH, 1039 01:06:02,720 --> 01:06:06,420 and needed a place to talk about it somewhere in the course. 1040 01:06:06,420 --> 01:06:06,920 All right. 1041 01:06:06,920 --> 01:06:10,440 Now back to photosynthesis. 1042 01:06:10,440 --> 01:06:14,090 Now, for photosynthesis, this reduced 1043 01:06:14,090 --> 01:06:17,000 NADPH molecule's of course useful, 1044 01:06:17,000 --> 01:06:19,970 because it can serve as a source of electrons 1045 01:06:19,970 --> 01:06:23,660 to donate carbons to reduce CO2. 1046 01:06:23,660 --> 01:06:29,330 So if you're going to reduce CO2 to make carbohydrate or fat, 1047 01:06:29,330 --> 01:06:30,860 you need this source of electrons. 1048 01:06:30,860 --> 01:06:38,020 Those can come from NADPH, which can be oxidized to NADP+. 1049 01:06:38,020 --> 01:06:40,120 And so perhaps at this point, you've 1050 01:06:40,120 --> 01:06:42,040 already guessed or maybe already know 1051 01:06:42,040 --> 01:06:48,340 that photosynthesis uses the favorable electron transfer 1052 01:06:48,340 --> 01:06:52,690 from water to generate NADPH. 1053 01:06:52,690 --> 01:06:56,540 And to do that, you use the energy of the sun. 1054 01:06:56,540 --> 01:07:00,760 Now, of course, if you're going to do a favorable electron 1055 01:07:00,760 --> 01:07:06,220 transfer, that can be coupled, just like we talked about 1056 01:07:06,220 --> 01:07:12,640 for oxidative phosphorylation, to make delta psi/delta pH. 1057 01:07:12,640 --> 01:07:15,550 And if we can use that favorable electron transfer 1058 01:07:15,550 --> 01:07:18,460 to make delta psi/delta pH, we can also 1059 01:07:18,460 --> 01:07:23,920 use it to do other work, including making ATP exactly 1060 01:07:23,920 --> 01:07:28,540 as we described for oxidative phosphorylation. 1061 01:07:28,540 --> 01:07:32,500 And at the highest level, this is exactly how photosynthesis 1062 01:07:32,500 --> 01:07:35,920 traps solar energy and uses that energy 1063 01:07:35,920 --> 01:07:41,820 to both remake reduced carbon and generates 1064 01:07:41,820 --> 01:07:44,970 a battery for the photosynthetic organism that 1065 01:07:44,970 --> 01:07:48,390 can be used to make ATP or do other work, 1066 01:07:48,390 --> 01:07:52,410 provided that sun is available. 1067 01:07:52,410 --> 01:07:55,770 And so classically, photosynthesis 1068 01:07:55,770 --> 01:08:00,090 can really be separated into, I guess, two distinct processes. 1069 01:08:00,090 --> 01:08:00,940 All right? 1070 01:08:00,940 --> 01:08:09,240 So you have this process of taking solar energy, 1071 01:08:09,240 --> 01:08:13,680 using that solar energy to charge up 1072 01:08:13,680 --> 01:08:18,210 a battery, delta psi/delta pH, which of course can be used 1073 01:08:18,210 --> 01:08:22,620 to generate ATP or do other work, and at the same time 1074 01:08:22,620 --> 01:08:27,630 gives you an electron donor, NADPH, 1075 01:08:27,630 --> 01:08:32,010 that is useful to generate reduced carbon. 1076 01:08:32,010 --> 01:08:40,319 Once you generate, high ATP/ADP ratio, high NADPH/NADP ratio. 1077 01:08:40,319 --> 01:08:45,120 Now we can use that to be couple to do something 1078 01:08:45,120 --> 01:08:47,370 else unfavorable. 1079 01:08:47,370 --> 01:08:51,870 And that would be something like reduced CO2 1080 01:08:51,870 --> 01:08:54,000 to generate a carbohydrate. 1081 01:08:57,310 --> 01:09:00,580 And typically, when we talk about photosynthesis, 1082 01:09:00,580 --> 01:09:02,350 the way it's often described is how you 1083 01:09:02,350 --> 01:09:04,210 reduced CO2 to carbohydrate. 1084 01:09:04,210 --> 01:09:05,470 And we'll do that too. 1085 01:09:05,470 --> 01:09:09,399 But of course, we'll then see once you have carbohydrates, 1086 01:09:09,399 --> 01:09:11,800 obviously, it's just further reduction of carbon 1087 01:09:11,800 --> 01:09:16,569 to make fatty acids and lipids. 1088 01:09:16,569 --> 01:09:18,460 We'll discuss later how we can use 1089 01:09:18,460 --> 01:09:20,109 those to make amino acids, which would 1090 01:09:20,109 --> 01:09:21,800 allow you to make proteins. 1091 01:09:21,800 --> 01:09:28,890 And, of course, we can also store those carbohydrates 1092 01:09:28,890 --> 01:09:32,920 as disaccharides or polysaccharides, storage 1093 01:09:32,920 --> 01:09:35,140 sugars, which is what plants do. 1094 01:09:35,140 --> 01:09:37,930 They make starch, they make sucrose, et cetera, 1095 01:09:37,930 --> 01:09:41,229 as a way to store that reduce carbon 1096 01:09:41,229 --> 01:09:44,439 so that they can have a source of energy, 1097 01:09:44,439 --> 01:09:47,950 do the oxidation reactions, to deal with their energetic needs 1098 01:09:47,950 --> 01:09:50,710 when the sun is not shining. 1099 01:09:50,710 --> 01:09:52,189 All right. 1100 01:09:52,189 --> 01:09:58,150 So let's break down these two processes. 1101 01:09:58,150 --> 01:10:01,510 And so to just be very explicit, the first part 1102 01:10:01,510 --> 01:10:06,400 is basically trapping energy to make a battery. 1103 01:10:06,400 --> 01:10:07,540 All right? 1104 01:10:07,540 --> 01:10:11,830 And so this means that we need to generate delta psi/delta 1105 01:10:11,830 --> 01:10:16,810 pH via favorable electron transport, exactly what we 1106 01:10:16,810 --> 01:10:17,960 describe for OXPHOS. 1107 01:10:17,960 --> 01:10:20,200 But, of course, photosynthesis was first. 1108 01:10:20,200 --> 01:10:23,440 This would then allow us to use ATP to fill our energy needs. 1109 01:10:23,440 --> 01:10:27,550 And we get NADPH if we set it up in the right way, which is 1110 01:10:27,550 --> 01:10:30,360 important for carbon reduction. 1111 01:10:30,360 --> 01:10:34,170 Then we need to use NADPH and ATP via pathways 1112 01:10:34,170 --> 01:10:38,370 to describe how we can use that to reduce CO2, and ultimately 1113 01:10:38,370 --> 01:10:42,940 produce glucose, which allows us to store energy for later. 1114 01:10:42,940 --> 01:10:43,440 All right. 1115 01:10:43,440 --> 01:10:46,890 So that is, at a very high level, what photosynthesis is. 1116 01:10:46,890 --> 01:10:50,100 And, of course, this occurs in plants. 1117 01:10:50,100 --> 01:10:51,730 All right? 1118 01:10:51,730 --> 01:10:53,680 But perhaps more importantly, it also 1119 01:10:53,680 --> 01:11:10,190 occurs in algae and other unicellular eukaryotes 1120 01:11:10,190 --> 01:11:17,670 as well as in prokaryotes-- photosynthetic prokaryotes, 1121 01:11:17,670 --> 01:11:19,050 bacteria. 1122 01:11:19,050 --> 01:11:21,870 And, of course, photosynthetic bacteria, 1123 01:11:21,870 --> 01:11:24,210 unicellular eukaryotes, algae-- 1124 01:11:24,210 --> 01:11:26,890 these really are the heroes of life. 1125 01:11:26,890 --> 01:11:30,450 These are the things that life started from, and generated 1126 01:11:30,450 --> 01:11:32,670 the oxygenated atmosphere, and ultimately 1127 01:11:32,670 --> 01:11:35,280 led to the evolution of higher plants, 1128 01:11:35,280 --> 01:11:39,660 but also enabled the formation of animal life, 1129 01:11:39,660 --> 01:11:44,880 and really anything that lives off of eating other organisms. 1130 01:11:44,880 --> 01:11:47,610 Just a few facts about photosynthesis 1131 01:11:47,610 --> 01:11:51,310 because it's really quite an amazing process-- 1132 01:11:51,310 --> 01:11:56,850 and so about 10 to the 17th kilojoules, at least 1133 01:11:56,850 --> 01:12:04,730 by some estimate, of energy is harvested on our planet 1134 01:12:04,730 --> 01:12:07,400 from photosynthesis every year. 1135 01:12:07,400 --> 01:12:09,950 Just to give you a perspective of what that number means, 1136 01:12:09,950 --> 01:12:14,180 that is 10 times the worldwide energy use 1137 01:12:14,180 --> 01:12:15,530 by all people on Earth. 1138 01:12:15,530 --> 01:12:18,680 So if you add up all the energy that we as humans use 1139 01:12:18,680 --> 01:12:23,210 in the world, it is tenfold less than the amount of energy 1140 01:12:23,210 --> 01:12:27,630 that is harvested by photosynthesis, 1141 01:12:27,630 --> 01:12:31,720 by photosynthetic organisms around the world. 1142 01:12:31,720 --> 01:12:36,550 Photosynthetic organisms, when they do this, consume CO2. 1143 01:12:36,550 --> 01:12:37,330 All right? 1144 01:12:37,330 --> 01:12:40,060 So that is removing CO2, the greenhouse 1145 01:12:40,060 --> 01:12:43,820 gas from our atmosphere. 1146 01:12:43,820 --> 01:12:46,970 And this occurs on a massive scale. 1147 01:12:46,970 --> 01:12:51,700 So this is better than any man-made engineering solution 1148 01:12:51,700 --> 01:12:55,040 that we have come up with by far. 1149 01:12:55,040 --> 01:12:58,100 And I think this really illustrates 1150 01:12:58,100 --> 01:13:01,640 the awesome power of biology to solve 1151 01:13:01,640 --> 01:13:05,790 some of the biggest problems that face our society today. 1152 01:13:05,790 --> 01:13:08,930 So world energy problems-- 1153 01:13:08,930 --> 01:13:16,250 photosynthesis harnesses a ton of energy. 1154 01:13:16,250 --> 01:13:18,230 Global warming, greenhouse gases-- 1155 01:13:18,230 --> 01:13:20,990 photosynthesis, better than almost anything 1156 01:13:20,990 --> 01:13:26,190 at dealing with these CO2 that is produced. 1157 01:13:26,190 --> 01:13:31,567 However, this also illustrates what some of the problems are 1158 01:13:31,567 --> 01:13:33,650 and what some of the issues are with fossil fuels, 1159 01:13:33,650 --> 01:13:35,600 because what are fossil fuels. 1160 01:13:35,600 --> 01:13:37,790 Well, they're effectively reduced 1161 01:13:37,790 --> 01:13:41,900 carbon that is trapped in the ground somewhere, that is 1162 01:13:41,900 --> 01:13:44,430 effectively old photosynthesis. 1163 01:13:44,430 --> 01:13:50,000 So these are organisms in many, many, many, many years ago 1164 01:13:50,000 --> 01:13:53,750 that did photosynthesis, made reduced carbon that 1165 01:13:53,750 --> 01:13:56,480 was effectively trapped in the ground. 1166 01:13:56,480 --> 01:14:01,970 And so the fact that photosynthesis transducers 10 1167 01:14:01,970 --> 01:14:06,380 to 17th kilojoules of energy per year and we're using 1/10 1168 01:14:06,380 --> 01:14:09,620 of that amount of energy, if that's all coming from fossil 1169 01:14:09,620 --> 01:14:13,790 fuels, that means we're using 1/10 of a year's worth 1170 01:14:13,790 --> 01:14:15,470 of photosynthesis-- and, of course, 1171 01:14:15,470 --> 01:14:18,290 a year is not entirely stored in the ground-- 1172 01:14:18,290 --> 01:14:18,980 per year. 1173 01:14:18,980 --> 01:14:20,240 So that's a lot of energy. 1174 01:14:20,240 --> 01:14:24,710 And it really comes down to why people say fossil fuels can't 1175 01:14:24,710 --> 01:14:30,890 last forever, and also why the net release of these things 1176 01:14:30,890 --> 01:14:33,560 can change our atmosphere, because, of course, 1177 01:14:33,560 --> 01:14:37,370 once upon a time, there was much more CO2 in the atmosphere. 1178 01:14:37,370 --> 01:14:39,890 And photosynthesis, over millennia, 1179 01:14:39,890 --> 01:14:42,980 trapping these things as fossil fuels really 1180 01:14:42,980 --> 01:14:46,640 has lowered CO2 in our atmosphere to the levels 1181 01:14:46,640 --> 01:14:51,320 that they exist today, and how, basically, 1182 01:14:51,320 --> 01:14:54,950 burning fossil fuels on the scale that we are 1183 01:14:54,950 --> 01:14:59,370 is able to change our atmosphere. 1184 01:14:59,370 --> 01:15:01,880 So you can view this both as glass half full and glass 1185 01:15:01,880 --> 01:15:03,080 half empty. 1186 01:15:03,080 --> 01:15:05,330 It really says understanding photosynthesis 1187 01:15:05,330 --> 01:15:08,430 could be a key to solving some of these problems. 1188 01:15:08,430 --> 01:15:11,780 But it also points out why some of these things 1189 01:15:11,780 --> 01:15:14,200 might be problems to begin with. 1190 01:15:14,200 --> 01:15:15,310 All right. 1191 01:15:15,310 --> 01:15:20,380 Now we want to discuss first how photosynthesis traps energy. 1192 01:15:20,380 --> 01:15:24,190 And you'll see that it is similar to oxidative 1193 01:15:24,190 --> 01:15:29,060 phosphorylation in many, many ways. 1194 01:15:29,060 --> 01:15:33,430 So it's going to occur at a membrane. 1195 01:15:33,430 --> 01:15:35,250 All right? 1196 01:15:35,250 --> 01:15:44,220 It's going to involve a series of protein complexes 1197 01:15:44,220 --> 01:15:46,590 where we're going to favor, we're 1198 01:15:46,590 --> 01:15:49,320 going to couple favorable electron 1199 01:15:49,320 --> 01:15:53,760 transport along a electron transport chain 1200 01:15:53,760 --> 01:15:56,670 to favor proton pumping. 1201 01:15:56,670 --> 01:16:01,730 That is going to generate a membrane potential delta 1202 01:16:01,730 --> 01:16:02,490 psi/delta pH. 1203 01:16:05,210 --> 01:16:07,190 In order to do that, of course, we 1204 01:16:07,190 --> 01:16:09,560 need favorable electron transport 1205 01:16:09,560 --> 01:16:14,360 from something that is lower standard reduction 1206 01:16:14,360 --> 01:16:18,350 potential to something that is higher standard reduction 1207 01:16:18,350 --> 01:16:18,890 potential. 1208 01:16:18,890 --> 01:16:21,200 That is what is going to make it favorable, 1209 01:16:21,200 --> 01:16:24,050 allow proton pumping. 1210 01:16:24,050 --> 01:16:28,370 That, when it generates delta psi/delta pH, 1211 01:16:28,370 --> 01:16:35,560 we can now harness that as a way to synthesize 1212 01:16:35,560 --> 01:16:42,296 ATP or do other work, as we've now talked about a lot. 1213 01:16:42,296 --> 01:16:44,046 Looks a lot like oxidative phosphorylation 1214 01:16:44,046 --> 01:16:45,350 and it makes sense. 1215 01:16:45,350 --> 01:16:49,460 Oxidative phosphorylation came from photosynthesis. 1216 01:16:49,460 --> 01:16:52,550 However, there are some really big differences 1217 01:16:52,550 --> 01:16:54,020 between the two. 1218 01:16:54,020 --> 01:16:57,260 And that is in photosynthesis, as I've already alluded 1219 01:16:57,260 --> 01:17:03,710 to, the electron donor is going to be water, 1220 01:17:03,710 --> 01:17:11,960 which will generate oxygen. And the electron acceptor is NADP+, 1221 01:17:11,960 --> 01:17:14,710 which will generate NADPH. 1222 01:17:19,230 --> 01:17:22,080 And you might now be asking, well, how on Earth 1223 01:17:22,080 --> 01:17:23,790 can this be possible? 1224 01:17:23,790 --> 01:17:33,830 Because, remember, the standard reduction potential of NADH 1225 01:17:33,830 --> 01:17:38,630 is less than the standard reduction potential 1226 01:17:38,630 --> 01:17:42,320 of our oxygen water pair. 1227 01:17:42,320 --> 01:17:46,430 And it's really this that makes the change 1228 01:17:46,430 --> 01:17:49,580 in standard reduction potential positive, which 1229 01:17:49,580 --> 01:17:53,790 means that delta G is going to be negative, 1230 01:17:53,790 --> 01:17:56,150 which is why electron transport and oxidative 1231 01:17:56,150 --> 01:17:58,700 phosphorylation is favorable and can be coupled 1232 01:17:58,700 --> 01:18:02,510 to make delta psi/delta pH. 1233 01:18:02,510 --> 01:18:06,800 Now, adding a phosphate to NAD/NADH 1234 01:18:06,800 --> 01:18:09,800 does not change its standard reduction potential 1235 01:18:09,800 --> 01:18:12,980 in a way that is at all meaningful to impact 1236 01:18:12,980 --> 01:18:14,270 what goes on. 1237 01:18:14,270 --> 01:18:19,700 And so this is really where energy input from the sun 1238 01:18:19,700 --> 01:18:21,410 becomes critical. 1239 01:18:21,410 --> 01:18:28,640 And that is-- it is this light energy from a photon that 1240 01:18:28,640 --> 01:18:32,820 in the end makes water a good electron donor, 1241 01:18:32,820 --> 01:18:36,920 such that all of these other things are satisfied that 1242 01:18:36,920 --> 01:18:41,180 electron transport is favorable going from lower to higher 1243 01:18:41,180 --> 01:18:47,090 standard reduction potential with NADP+ ultimately being 1244 01:18:47,090 --> 01:18:51,590 the final electron acceptor, such that the change 1245 01:18:51,590 --> 01:18:55,340 in standard production potential is positive. 1246 01:18:55,340 --> 01:19:00,385 And that really is the magic of photosynthesis. 1247 01:19:00,385 --> 01:19:01,760 And, of course, once you do that, 1248 01:19:01,760 --> 01:19:03,830 now you have delta psi/delta pH. 1249 01:19:03,830 --> 01:19:06,200 You can use that to make ATP, do other work 1250 01:19:06,200 --> 01:19:08,060 for the photosynthetic organism. 1251 01:19:08,060 --> 01:19:10,370 And you also get NADPH, which is an electron 1252 01:19:10,370 --> 01:19:12,800 donor for carbon reduction. 1253 01:19:12,800 --> 01:19:19,370 And the net result is that you get this familiar formula 1254 01:19:19,370 --> 01:19:22,340 that you learned in grade school for photosynthesis-- 1255 01:19:22,340 --> 01:19:32,290 CO2 plus water with light goes to oxygen plus carbohydrate, 1256 01:19:32,290 --> 01:19:37,800 which is, of course, the exact opposite of combustion. 1257 01:19:37,800 --> 01:19:43,740 We talked a lot about how burning wood combustion is 1258 01:19:43,740 --> 01:19:46,710 the opposite reaction that's favorable-- lots of energy 1259 01:19:46,710 --> 01:19:48,130 released. 1260 01:19:48,130 --> 01:19:52,650 And so the energy input here is obviously 1261 01:19:52,650 --> 01:19:58,440 the opposite of that to do this opposite of combustion 1262 01:19:58,440 --> 01:20:00,510 and stores lots of energy. 1263 01:20:00,510 --> 01:20:05,830 All right, so again, I want to say why 1264 01:20:05,830 --> 01:20:08,920 use water as an electron donor? 1265 01:20:08,920 --> 01:20:10,180 Well, because it's abundant. 1266 01:20:10,180 --> 01:20:13,360 There's nothing particularly special about water other 1267 01:20:13,360 --> 01:20:18,070 than its abundance, just like we talked about in oxygen is not 1268 01:20:18,070 --> 01:20:19,990 really being special other than that it's 1269 01:20:19,990 --> 01:20:23,170 a good electron acceptor and abundant. 1270 01:20:23,170 --> 01:20:26,380 You could obviously build systems that do something other 1271 01:20:26,380 --> 01:20:28,670 than oxygen and water. 1272 01:20:28,670 --> 01:20:31,000 And so there are extremophiles out there 1273 01:20:31,000 --> 01:20:37,360 that instead do hydrogen sulfide to make 1274 01:20:37,360 --> 01:20:40,140 reduced sulfur as another-- 1275 01:20:42,900 --> 01:20:49,470 sorry, oxidize sulfur as a way of doing photosynthesis. 1276 01:20:49,470 --> 01:20:52,170 Exact same concepts would apply is what 1277 01:20:52,170 --> 01:20:55,140 we're going to talk about here. 1278 01:20:55,140 --> 01:20:55,650 All right. 1279 01:20:55,650 --> 01:21:00,450 Now, the parts of photosynthesis, 1280 01:21:00,450 --> 01:21:03,880 as you might guess, are spatially distinct. 1281 01:21:03,880 --> 01:21:12,900 That is, we're going to generate delta psi/delta pH and NADPH. 1282 01:21:12,900 --> 01:21:15,690 Those are basically this electron 1283 01:21:15,690 --> 01:21:19,960 transport chain reactions, charge the battery, et cetera. 1284 01:21:19,960 --> 01:21:30,080 And then imagine if we're going to fix CO2 as carbohydrate. 1285 01:21:30,080 --> 01:21:32,480 Well, effectively that's more like what we talked about 1286 01:21:32,480 --> 01:21:34,130 with gluconeogenesis. 1287 01:21:34,130 --> 01:21:38,960 That's the chemical reactions that we just 1288 01:21:38,960 --> 01:21:42,440 have to build a pathway for, like gluconeogenesis. 1289 01:21:42,440 --> 01:21:44,480 Now these two things are often referred 1290 01:21:44,480 --> 01:21:56,210 to as the light and dark reactions of photosynthesis. 1291 01:21:56,210 --> 01:21:59,360 And that's because the first set of reactions 1292 01:21:59,360 --> 01:22:03,200 generating the membrane potential and making NADPH 1293 01:22:03,200 --> 01:22:07,430 requires solar energy inputs or requires light from the sun. 1294 01:22:07,430 --> 01:22:13,400 This fixing of CO2, which will just use electrons from NADPH 1295 01:22:13,400 --> 01:22:16,268 to reduce CO2, can occur-- 1296 01:22:16,268 --> 01:22:17,810 of course, it can occur in the light. 1297 01:22:17,810 --> 01:22:19,020 But it doesn't need light. 1298 01:22:19,020 --> 01:22:20,960 So it can occur in the light or the dark, 1299 01:22:20,960 --> 01:22:23,550 hence called the dark reactions. 1300 01:22:23,550 --> 01:22:28,850 And so these light reactions can really be broken up into water 1301 01:22:28,850 --> 01:22:39,050 plus NADP+ goes to NADPH plus oxygen. 1302 01:22:39,050 --> 01:22:50,870 And the dark reactions are NADPH plus CO2 goes to carbohydrate 1303 01:22:50,870 --> 01:22:58,850 plus NADP+, which of course gives us our net-- 1304 01:22:58,850 --> 01:23:07,950 water plus CO2 goes to carbohydrate plus oxygen. 1305 01:23:07,950 --> 01:23:10,860 And in the next lecture, what we'll talk about is 1306 01:23:10,860 --> 01:23:13,560 we'll go through the details of how 1307 01:23:13,560 --> 01:23:17,520 these light and dark reactions work. 1308 01:23:17,520 --> 01:23:20,310 But when we do so, bear in mind that we're really 1309 01:23:20,310 --> 01:23:25,350 going to follow all of the concepts and rules 1310 01:23:25,350 --> 01:23:30,120 that we talked about for past thermodynamic considerations 1311 01:23:30,120 --> 01:23:32,610 of pathways as well as what we've 1312 01:23:32,610 --> 01:23:35,820 been discussing about how oxidative phosphorylation 1313 01:23:35,820 --> 01:23:36,570 works. 1314 01:23:36,570 --> 01:23:38,120 Thanks.