WEBVTT 00:00:14.790 --> 00:00:21.150 PROFESSOR: Let's talk about what was probably the first 00:00:21.150 --> 00:00:24.442 energy producing system that evolved. 00:00:28.100 --> 00:00:32.840 The thought is when the earth first formed and the first 00:00:32.840 --> 00:00:36.530 primitive organisms, perhaps resembling a present-day 00:00:36.530 --> 00:00:39.550 bacterium in some way came out, there were a lot of 00:00:39.550 --> 00:00:41.970 organic compounds that had been aided by lightning 00:00:41.970 --> 00:00:45.410 strikes and cosmic radiations triggering chemical 00:00:45.410 --> 00:00:46.615 reactions and so on. 00:00:46.615 --> 00:00:49.980 So there was food around, but they depleted those resources 00:00:49.980 --> 00:00:51.820 in the same way we're depleting the petroleum 00:00:51.820 --> 00:00:53.180 resources right now. 00:00:53.180 --> 00:00:59.950 If life was going to continue, somehow a way had to be found 00:00:59.950 --> 00:01:01.790 to make energy. 00:01:01.790 --> 00:01:09.220 Glycolysis, it looks kind of complicated. 00:01:09.220 --> 00:01:13.760 It takes a molecule of sugar and then there are a series of 00:01:13.760 --> 00:01:21.060 10 chemical reactions, each catalyzed by a separate enzyme 00:01:21.060 --> 00:01:42.940 that give two molecules of this, molecules of pyruvate, 00:01:42.940 --> 00:01:51.990 plus two ATPs, plus two NADHs. 00:01:51.990 --> 00:01:55.320 Which tells you there must have been some kind of 00:01:55.320 --> 00:01:59.020 oxidation step as part of this sequence of events, because 00:01:59.020 --> 00:02:06.190 electrons got taken off and got stashed on this NADH. 00:02:06.190 --> 00:02:09.250 There are a couple of things that are important about this. 00:02:09.250 --> 00:02:14.340 One is its a pathway. 00:02:14.340 --> 00:02:19.600 It evolved probably 3.7 billion years ago or sometime, 00:02:19.600 --> 00:02:20.980 nobody really knows. 00:02:20.980 --> 00:02:22.990 But a long time ago. 00:02:22.990 --> 00:02:26.000 It's pretty much universal. 00:02:26.000 --> 00:02:29.037 Not perfectly so, but it's in bacteria, it's in 00:02:29.037 --> 00:02:31.045 yeast, it's in humans. 00:02:37.190 --> 00:02:45.220 And another really important thing is that it evolved early 00:02:45.220 --> 00:02:49.120 in the evolution of earth, so it evolved when there was no 00:02:49.120 --> 00:02:50.580 oxygen around. 00:02:50.580 --> 00:02:54.440 So it's a way of making energy from glucose in 00:02:54.440 --> 00:02:56.740 the absence of oxygen. 00:02:56.740 --> 00:03:00.280 Which is a really important thing as you'll 00:03:00.280 --> 00:03:01.920 see as we go along. 00:03:05.520 --> 00:03:10.080 You're not going to have to memorize this pathway. 00:03:10.080 --> 00:03:13.200 We'll give it to you if you need it. 00:03:13.200 --> 00:03:17.070 But you're going to need to understand its implications. 00:03:17.070 --> 00:03:19.300 And just let me point out a couple of things. 00:03:19.300 --> 00:03:23.340 You're going to see a sequence of 10 chemical transformations 00:03:23.340 --> 00:03:25.850 that in the end are going to end up with a couple of 00:03:25.850 --> 00:03:27.910 pyruvates being produced. 00:03:27.910 --> 00:03:30.220 And I'll try to explain to you why you 00:03:30.220 --> 00:03:35.010 should care about this. 00:03:35.010 --> 00:03:37.940 There's a concept that you're familiar with, that if you 00:03:37.940 --> 00:03:40.170 want to make something and you get a little start up company, 00:03:40.170 --> 00:03:41.980 what's the very first thing you have to do? 00:03:41.980 --> 00:03:44.060 You actually have to make an investment 00:03:44.060 --> 00:03:45.030 before you can get going. 00:03:45.030 --> 00:03:47.370 And you're out looking for venture capital things. 00:03:47.370 --> 00:03:49.840 Well one of the odd things about this, here's probably 00:03:49.840 --> 00:03:53.380 the first sequence of reactions that arose on earth 00:03:53.380 --> 00:03:56.520 within some organism and enabled that organism to make 00:03:56.520 --> 00:03:57.490 energy out of glucose. 00:03:57.490 --> 00:03:58.810 And look, the first thing that happens. 00:03:58.810 --> 00:04:02.270 Trying to make ATP, the very first thing it does is it 00:04:02.270 --> 00:04:04.370 spends an ATP. 00:04:04.370 --> 00:04:10.020 And it takes glucose, and it makes glucose 6-phosphate. 00:04:10.020 --> 00:04:11.420 Go down a couple of steps. 00:04:11.420 --> 00:04:16.769 There's an enzyme that takes another molecule of ATP. 00:04:16.769 --> 00:04:19.110 And now you've got this point. 00:04:19.110 --> 00:04:22.130 You're at fructose with two phosphates on it. 00:04:22.130 --> 00:04:25.560 If this was your venture capital, we'd say, guys, how 00:04:25.560 --> 00:04:26.400 about some product? 00:04:26.400 --> 00:04:29.760 Stop spending, stop spending money. 00:04:29.760 --> 00:04:33.520 But at this point then, this is a 6-carbon sugar. 00:04:33.520 --> 00:04:40.970 And it gets split into two 3-carbon compounds that are 00:04:40.970 --> 00:04:42.720 going back and forth. 00:04:42.720 --> 00:04:43.550 Oh I can see it. 00:04:43.550 --> 00:04:44.800 It's over there, OK. 00:04:52.800 --> 00:04:55.740 In equilibrium over here. 00:04:55.740 --> 00:04:59.650 And this particular 3-carbon compound then 00:04:59.650 --> 00:05:04.820 goes on to be oxidized. 00:05:04.820 --> 00:05:08.030 You get the production of NADH. 00:05:08.030 --> 00:05:11.570 And at that point, this molecule has a lot of energy 00:05:11.570 --> 00:05:14.430 stored in it, and in the next transformation this cell is 00:05:14.430 --> 00:05:16.960 able to make two ATPs. 00:05:16.960 --> 00:05:18.820 And it gets back the initial investment. 00:05:18.820 --> 00:05:21.320 It goes all the way through the rest of the pathway. 00:05:21.320 --> 00:05:25.650 And the very last step, you get two more ATPs back. 00:05:25.650 --> 00:05:28.000 There's your net yield. 00:05:28.000 --> 00:05:37.610 So what you get out of this are 4ATP+2NADH, and your 00:05:37.610 --> 00:05:41.380 investment was two ATPs. 00:05:41.380 --> 00:05:45.720 So your net 2ATP+NADH. 00:05:50.430 --> 00:05:52.640 Why is this cell going and doing these initial 00:05:52.640 --> 00:05:53.240 investments? 00:05:53.240 --> 00:05:58.280 Well if we look at the changes in free energy associated with 00:05:58.280 --> 00:06:01.110 what's going on, there's glucose up in the upper left 00:06:01.110 --> 00:06:04.710 starting up there, and there's pyruvate down there. 00:06:04.710 --> 00:06:08.010 So you're going energetically downhill in the end. 00:06:08.010 --> 00:06:11.350 So this is a sequence of events that, in principle, you 00:06:11.350 --> 00:06:13.660 should be able to get some energy out of it. 00:06:13.660 --> 00:06:17.570 But for reasons that may seem obscure to you at this point, 00:06:17.570 --> 00:06:21.040 before it gets to the point of making energy, it undergoes a 00:06:21.040 --> 00:06:24.590 set of transformations that's pushing the reaction. 00:06:24.590 --> 00:06:28.870 It requires the reactants to go energetically uphill, i.e. 00:06:28.870 --> 00:06:31.590 in an unfavorable direction. 00:06:31.590 --> 00:06:36.960 So what the cell does is, by coupling ATP hydrolysis to 00:06:36.960 --> 00:06:40.220 this step, it makes that reaction go. 00:06:40.220 --> 00:06:43.780 Here's another unfavorable one that makes that one go by 00:06:43.780 --> 00:06:46.360 coupling ATP hydrolysis to it. 00:06:46.360 --> 00:06:50.590 This is an uphill reaction, but look over here. 00:06:50.590 --> 00:06:54.710 This is an immensely favorable reaction that goes essentially 00:06:54.710 --> 00:06:55.150 to completion. 00:06:55.150 --> 00:06:56.570 It goes all the way. 00:06:56.570 --> 00:06:59.480 So that means this product is just being continually taken 00:06:59.480 --> 00:07:02.780 out of the system, so the equilibrium is basically being 00:07:02.780 --> 00:07:06.520 pulled over the edge by the removal of that product. 00:07:06.520 --> 00:07:08.270 This is where the oxidation takes place. 00:07:08.270 --> 00:07:11.580 You get the NADH made right there. 00:07:11.580 --> 00:07:15.770 And it's finally down here where you've got to lose. 00:07:15.770 --> 00:07:21.080 This transformation gives you two ATPs and later there's 00:07:21.080 --> 00:07:21.930 another one. 00:07:21.930 --> 00:07:25.690 Let me just give you a sense of why you 00:07:25.690 --> 00:07:28.540 get ATP at that step. 00:07:28.540 --> 00:07:36.460 The compound that you have at that point is 1, 3 00:07:36.460 --> 00:07:37.710 diphosphoglycerate. 00:07:49.860 --> 00:07:52.710 Or sometimes this is called bis, is also 00:07:52.710 --> 00:07:53.930 used to describe this. 00:07:53.930 --> 00:07:56.750 But what is this compound? 00:07:56.750 --> 00:07:59.770 It's a 3-carbon compound. 00:08:11.870 --> 00:08:18.020 So glycerate is basically an oxidized version of glycerol 00:08:18.020 --> 00:08:21.080 that has been oxidized up to a carboxyl acid. 00:08:21.080 --> 00:08:26.230 So this is a mixed anhydride between carboxyl acid and a 00:08:26.230 --> 00:08:27.400 phosphate ion. 00:08:27.400 --> 00:08:31.210 So that's a very reactive and unstable compound. 00:08:31.210 --> 00:08:33.980 And the other thing that the cell has succeeded in doing by 00:08:33.980 --> 00:08:36.669 all of these transformations is it's got these two 00:08:36.669 --> 00:08:39.760 phosphates with all their negative charges in. 00:08:39.760 --> 00:08:43.010 So this is a compound that would very much like to move 00:08:43.010 --> 00:08:46.230 to a lower energy. 00:08:46.230 --> 00:08:51.000 So you can get rid of this phosphate and move to an 00:08:51.000 --> 00:08:54.320 energy level, use that energy to make ATP. 00:08:54.320 --> 00:08:58.240 And there's a similar kind of logic that explains why you 00:08:58.240 --> 00:09:01.510 get energy out of the final step when you look at it. 00:09:01.510 --> 00:09:07.760 So there's several points, I guess, to make out of this. 00:09:07.760 --> 00:09:10.330 One is its pathway. 00:09:10.330 --> 00:09:15.380 None of these reactions make any particular sense by 00:09:15.380 --> 00:09:16.990 themselves. 00:09:16.990 --> 00:09:19.120 You could have a cell that knew how to do one of them and 00:09:19.120 --> 00:09:21.170 it would gain nothing. 00:09:21.170 --> 00:09:26.040 Unless you wanted to use the product to make something. 00:09:26.040 --> 00:09:28.720 This thing only makes sense, these reactions only make 00:09:28.720 --> 00:09:32.810 sense in the context of this 10 step pathway. 00:09:32.810 --> 00:09:37.200 And each step in that pathway we were looking at is 00:09:37.200 --> 00:09:40.080 catalyzed by a different enzyme. 00:09:40.080 --> 00:09:43.040 So for an organism to pull this off, the first one that 00:09:43.040 --> 00:09:49.670 did it had to collect in one cell all 10 of those enzymes. 00:09:49.670 --> 00:09:53.870 And probably there is the reason that this is such a 00:09:53.870 --> 00:09:55.720 complicated system. 00:09:55.720 --> 00:09:58.040 If you were sitting as a designer you might be able to 00:09:58.040 --> 00:10:03.080 come up now with a more efficient way to get ATP out. 00:10:03.080 --> 00:10:06.880 But what happened evolutionarily was some bug 00:10:06.880 --> 00:10:09.910 somewhere got all of these things together, and now 00:10:09.910 --> 00:10:11.210 suddenly it could make energy. 00:10:11.210 --> 00:10:15.360 So it had a huge advantage over everybody else. 00:10:15.360 --> 00:10:20.110 And once it took over, that system took over, then it 00:10:20.110 --> 00:10:22.820 became universal. 00:10:22.820 --> 00:10:25.440 Whether it was the best that ever could be designed, it 00:10:25.440 --> 00:10:29.100 doesn't matter, because it had an evolutionary edge. 00:10:29.100 --> 00:10:32.700 And that's so, to some extent, we're looking at a living 00:10:32.700 --> 00:10:34.120 fossil, biochemical. 00:10:34.120 --> 00:10:39.750 But it's in bacteria, it's in yeast, and it's going on 00:10:39.750 --> 00:10:41.320 inside of our body. 00:10:41.320 --> 00:10:44.790 Another principle that I think you can see here, which I've 00:10:44.790 --> 00:10:50.260 been trying to say, is in this case, the energy consuming 00:10:50.260 --> 00:10:54.030 reactions are driven by coupling them to the 00:10:54.030 --> 00:10:55.400 hydrolysis of ATP. 00:10:55.400 --> 00:10:58.740 The cell spends a bit of its energy money to get these 00:10:58.740 --> 00:11:01.440 intermediates, knowing that it's invest-- 00:11:01.440 --> 00:11:05.160 well not knowing, but at least conceptually anyway, knowing 00:11:05.160 --> 00:11:09.650 that it's going to get its investment back. 00:11:09.650 --> 00:11:13.400 And then the reactions that release energy are used to 00:11:13.400 --> 00:11:16.770 drive the synthesis of ATP. 00:11:16.770 --> 00:11:20.830 And you'll begin to see, we're going to just talk about some 00:11:20.830 --> 00:11:24.090 other aspects of this in just a minute. 00:11:24.090 --> 00:11:25.850 So, what do you think? 00:11:25.850 --> 00:11:28.850 You're the first bug and you've got this and nobody 00:11:28.850 --> 00:11:32.420 else can do it, so you can start charging away. 00:11:32.420 --> 00:11:33.360 What do we need to do? 00:11:33.360 --> 00:11:36.000 We just let this thing cycle away? 00:11:36.000 --> 00:11:37.780 The stuff that I had up there, is it going to work? 00:11:42.630 --> 00:11:43.460 There's a problem. 00:11:43.460 --> 00:11:44.820 Anybody see what the problem is? 00:11:51.350 --> 00:11:55.320 We're making two molecules of ATP and two molecules of NADH. 00:11:58.272 --> 00:12:00.240 Talk to the person beside you. 00:12:00.240 --> 00:12:04.394 Figure out why something else has to happen. 00:12:04.394 --> 00:12:05.876 Go ahead. 00:12:05.876 --> 00:12:07.358 See if you've got any ideas. 00:12:27.612 --> 00:12:29.290 We're going to keep doing this, over and 00:12:29.290 --> 00:12:31.070 over and over again. 00:12:31.070 --> 00:12:34.034 AUDIENCE: [INAUDIBLE]. 00:12:34.034 --> 00:12:35.516 Where's the first ATP? 00:12:35.516 --> 00:12:38.040 Where's the first ATP? 00:12:38.040 --> 00:12:39.990 PROFESSOR: OK, let's imagine for the-- 00:12:39.990 --> 00:12:43.880 I don't think this process could have invented ATP, it 00:12:43.880 --> 00:12:45.540 had to have been around, because many of the 00:12:45.540 --> 00:12:49.010 enzymes used it. 00:12:49.010 --> 00:12:51.940 What else is being used in this thing though? 00:12:57.160 --> 00:12:58.553 Did I hear NAD? 00:13:02.956 --> 00:13:06.610 To make this thing work, I have to keep taking NADs out 00:13:06.610 --> 00:13:09.400 of my pocket and putting it in the reaction, or it isn't 00:13:09.400 --> 00:13:12.040 going to go anywhere. 00:13:12.040 --> 00:13:14.870 So this isn't such a great invention at the moment. 00:13:14.870 --> 00:13:21.460 We have to do something to get the NADH back to NAD+ so we 00:13:21.460 --> 00:13:25.300 can do another molecule of glucose. 00:13:25.300 --> 00:13:25.835 You guys see? 00:13:25.835 --> 00:13:26.660 Do you see this? 00:13:26.660 --> 00:13:29.520 This is really, really an important consideration. 00:13:29.520 --> 00:13:36.160 So in order for cells to make energy using glycolysis in the 00:13:36.160 --> 00:13:39.150 absence of oxygen, which is when it evolved, they have to 00:13:39.150 --> 00:13:44.700 do something with that NADH or it's only going to use up the 00:13:44.700 --> 00:13:49.060 few molecules of NAD+ in the cell, and then it stops. 00:13:49.060 --> 00:13:52.570 And so there are two ways that nature's figured. 00:13:52.570 --> 00:13:54.820 Major ways nature had figured out how to do that. 00:13:59.160 --> 00:14:02.200 So here's a molecule of pyruvate. 00:14:05.730 --> 00:14:06.980 I got an extra. 00:14:12.250 --> 00:14:14.300 Something was nagging at me when I did this here. 00:14:17.370 --> 00:14:20.380 Sorry about that. 00:14:20.380 --> 00:14:24.570 It's always hard to see things when you're up at the board. 00:14:24.570 --> 00:14:24.900 OK. 00:14:24.900 --> 00:14:26.710 Molecule of pyruvate. 00:14:26.710 --> 00:14:30.270 There's a couple of solutions that have been arrived at. 00:14:30.270 --> 00:14:36.580 One is to take NADH, 2NADH, this is 2H+. 00:14:36.580 --> 00:14:47.620 Convert, make these back into 2NAD+, and to take those 00:14:47.620 --> 00:14:55.320 electrons and put them on the pyruvate to give this 00:14:55.320 --> 00:15:02.340 molecule, which is lactic acid. 00:15:02.340 --> 00:15:04.650 So by parking the electrons there, the cell is able to 00:15:04.650 --> 00:15:07.570 recycle the NADH. 00:15:07.570 --> 00:15:10.490 And lactic acid, we've run into that. 00:15:10.490 --> 00:15:15.250 That's why I showed you this picture of yogurt. 00:15:15.250 --> 00:15:19.530 The lactobacilli that make yogurt take the sugars that 00:15:19.530 --> 00:15:22.760 are present in milk and make them into lactic acid. 00:15:22.760 --> 00:15:24.850 And what's interesting in their case is they, even 00:15:24.850 --> 00:15:28.350 though there's oxygen around, they don't do respiration, 00:15:28.350 --> 00:15:30.100 which you'll see you can get more energy. 00:15:30.100 --> 00:15:35.500 They want it to get very acidic because that prevents 00:15:35.500 --> 00:15:37.120 their competitors from growing. 00:15:37.120 --> 00:15:40.570 And that's why you can leave yogurt sitting out on the 00:15:40.570 --> 00:15:42.900 tabletop and it'll be OK for quite a while. 00:15:42.900 --> 00:15:45.660 Whereas if you left some milk or something it'll go bad 00:15:45.660 --> 00:15:46.910 almost right away. 00:15:49.430 --> 00:15:51.650 Here's another example of when we run into it. 00:15:51.650 --> 00:15:55.400 When we do hard aerobic exercise, when you're running 00:15:55.400 --> 00:15:57.720 or skating really hard, things you see in the Olympics all 00:15:57.720 --> 00:16:03.125 the time, you deplete the oxygen supply in your blood 00:16:03.125 --> 00:16:05.580 when you do hard anaerobic exercise. 00:16:05.580 --> 00:16:10.770 And so the cells have the same problem of regenerating NADH. 00:16:10.770 --> 00:16:12.910 The way they solve it is they make the lactic acid. 00:16:12.910 --> 00:16:15.570 And that contributes to the sore muscles you feel after 00:16:15.570 --> 00:16:20.830 you've done hard anaerobic training. 00:16:20.830 --> 00:16:29.990 The other way of handling this is to take the 2NADH plus two 00:16:29.990 --> 00:16:39.460 hydrogen ions to make it into acetaldehyde, two 00:16:39.460 --> 00:16:40.710 acetaldehydes. 00:16:44.580 --> 00:16:46.834 Plus two CO2s. 00:16:46.834 --> 00:16:48.466 Oops, excuse me. 00:16:48.466 --> 00:16:51.430 Let's do this first. 00:16:51.430 --> 00:16:57.960 And then take the 2NADH plus the 2H+. 00:16:57.960 --> 00:17:02.740 Convert this to 2NAD+, and what we get out of this are 00:17:02.740 --> 00:17:11.470 two molecules of ethanol plus two molecules of CO2. 00:17:11.470 --> 00:17:16.510 Again, a process that's very similar to you, familiar to 00:17:16.510 --> 00:17:19.369 you, when I was showing you yeast growing. 00:17:19.369 --> 00:17:22.790 What yeast is doing is it's carrying out glycolysis and 00:17:22.790 --> 00:17:25.869 then it's taking those extra electrons, putting them on the 00:17:25.869 --> 00:17:29.755 pyruvate and making ethanol and carbon dioxide. 00:17:32.360 --> 00:17:35.520 I think there's a fermentation with what we call a 00:17:35.520 --> 00:17:36.750 fermentation with yeast. 00:17:36.750 --> 00:17:40.700 I think in that case they're making bourbon whiskey. 00:17:40.700 --> 00:17:44.130 Wine making, beer making, it's all the same thing. 00:17:44.130 --> 00:17:48.980 You have yeast, you're converting the sugars first to 00:17:48.980 --> 00:17:55.150 pyruvate, and then making ethanol and carboxylic acid. 00:17:55.150 --> 00:17:56.880 So anyway. 00:17:56.880 --> 00:18:01.320 There's no energy gain out of this, but these 00:18:01.320 --> 00:18:03.320 are important processes. 00:18:03.320 --> 00:18:04.570 They're called fermentation. 00:18:12.060 --> 00:18:16.590 And they can happen when there's no oxygen around. 00:18:16.590 --> 00:18:24.950 If you recall, there's a version of photosynthesis, 00:18:24.950 --> 00:18:27.660 what I called the second release of photosynthesis that 00:18:27.660 --> 00:18:31.280 began to evolve oxygen as a waste product. 00:18:31.280 --> 00:18:37.250 And then over the next ensuing millennia, the levels of 00:18:37.250 --> 00:18:40.920 oxygen slowly, slowly began to rise on earth. 00:18:40.920 --> 00:18:45.170 And as oxygen levels got to higher levels, and recall the 00:18:45.170 --> 00:18:48.990 Cambrian period, which is down on the fourth 00:18:48.990 --> 00:18:50.210 blackboard down there. 00:18:50.210 --> 00:18:52.890 We were only still even there half a billion years ago. 00:18:52.890 --> 00:18:55.840 We were only about 5% the present oxygen levels. 00:18:55.840 --> 00:19:02.690 But as oxygen levels arose, new metabolic opportunities 00:19:02.690 --> 00:19:03.485 became available. 00:19:03.485 --> 00:19:07.290 And in particular, cells were able to get at that energy 00:19:07.290 --> 00:19:10.160 which is stored in NADH. 00:19:10.160 --> 00:19:12.890 In the absence of oxygen, NADH is just a nuisance. 00:19:12.890 --> 00:19:14.390 You've got to get rid of it. 00:19:14.390 --> 00:19:16.010 But as you'll see in a minute, you can do something 00:19:16.010 --> 00:19:18.230 interesting if you have oxygen around. 00:19:18.230 --> 00:19:21.860 So just to look at this from a broad perspective, if we have 00:19:21.860 --> 00:19:26.260 glucose and we have all these little steps going along to 00:19:26.260 --> 00:19:35.490 give the two pyruvate, if there's, in the absence of 00:19:35.490 --> 00:19:50.380 oxygen, they get 2 lactate or we can get 2 ethanol, 2CO2. 00:19:53.320 --> 00:19:54.690 And in both cases, 2ATP. 00:19:59.240 --> 00:20:01.010 2ATP. 00:20:01.010 --> 00:20:06.710 These processes happening in the absence of oxygen to get 00:20:06.710 --> 00:20:11.010 rid of the, or at least not requiring oxygen in any case 00:20:11.010 --> 00:20:12.600 called fermentations. 00:20:12.600 --> 00:20:20.490 However, when oxygen is available, it became possible 00:20:20.490 --> 00:20:24.630 to evolve a new system for handling these pyruvates. 00:20:24.630 --> 00:20:29.650 We go into a biochemical cycle known as 00:20:29.650 --> 00:20:34.305 the citric acid cycle. 00:20:34.305 --> 00:20:36.920 And I'll say a word about this in a minute. 00:20:36.920 --> 00:20:44.250 Plus something else that's known as oxidative 00:20:44.250 --> 00:20:45.500 phosphorylation. 00:20:52.940 --> 00:20:56.100 This is also referred to as the respiratory chain. 00:21:01.180 --> 00:21:05.710 And what these two sets of processes together, enable the 00:21:05.710 --> 00:21:10.960 cell to take these two 3-carbon compounds and take 00:21:10.960 --> 00:21:15.120 them all the way down to six molecules of carbon dioxide, 00:21:15.120 --> 00:21:18.920 six molecules of water. 00:21:18.920 --> 00:21:29.010 And to make a net yield of 36 molecules of ATP. 00:21:29.010 --> 00:21:31.710 So if you go by fermentation a molecule of sugar 00:21:31.710 --> 00:21:33.650 gives you two ATPs. 00:21:33.650 --> 00:21:38.300 If you go by glycolysis and then follow it by respiration, 00:21:38.300 --> 00:21:39.130 you get 36. 00:21:39.130 --> 00:21:48.350 So respiration using oxygen, 18 times more efficient than 00:21:48.350 --> 00:21:50.160 by glycolysis. 00:21:50.160 --> 00:21:56.330 So in order to understand how this works though, we have to 00:21:56.330 --> 00:22:01.240 talk more about how you change from one form 00:22:01.240 --> 00:22:02.930 of energy to another. 00:22:02.930 --> 00:22:06.275 And it's interesting, although this process had to have 00:22:06.275 --> 00:22:10.310 evolved billions of years ago, it was only relatively 00:22:10.310 --> 00:22:14.790 recently that we understood the principal that was 00:22:14.790 --> 00:22:18.460 necessary for this kind of thing to happen. 00:22:18.460 --> 00:22:29.460 It's known as the Chemiosmotic Hypothesis. 00:22:29.460 --> 00:22:40.740 It was proposed by Peter Mitchell in 1961. 00:22:40.740 --> 00:22:44.550 He eventually got a Nobel Prize for it. 00:22:44.550 --> 00:22:49.780 It took quite a long time, it took more than 10 years for it 00:22:49.780 --> 00:22:50.630 to be accepted. 00:22:50.630 --> 00:22:54.310 In fact when I was in grad school in the mid '70s, people 00:22:54.310 --> 00:22:57.680 were still arguing whether this made sense or not. 00:22:57.680 --> 00:22:59.340 So here's the way it works. 00:22:59.340 --> 00:23:02.390 And we have to consider first three different forms of 00:23:02.390 --> 00:23:07.070 chemical energy that can be all interconverted. 00:23:07.070 --> 00:23:10.110 One of them is familiar to you, we've been talking about 00:23:10.110 --> 00:23:11.270 it all along. 00:23:11.270 --> 00:23:12.990 It's a chemical bond. 00:23:12.990 --> 00:23:15.745 Energy can be stored in a high energy bond. 00:23:15.745 --> 00:23:22.480 And if we break it to get ADP, an inorganic phosphate, we can 00:23:22.480 --> 00:23:24.135 release energy. 00:23:24.135 --> 00:23:26.600 However there's another way of storing energy as a 00:23:26.600 --> 00:23:27.850 concentration gradient. 00:23:30.980 --> 00:23:36.160 The principal here would be to have a barrier, which in this 00:23:36.160 --> 00:23:43.240 case is the cell membrane, and to have a high concentration 00:23:43.240 --> 00:23:49.460 of whatever it is on one side, and a low concentration on the 00:23:49.460 --> 00:23:50.710 other side. 00:23:50.710 --> 00:23:53.840 And there's energy stored in that. 00:23:53.840 --> 00:23:56.080 If you give it a chance it'll get to be the same 00:23:56.080 --> 00:23:58.240 concentration on both sides. 00:23:58.240 --> 00:24:01.950 And the trick is to have whatever the substance is, is 00:24:01.950 --> 00:24:09.920 to have a protein in the membrane that can permit this 00:24:09.920 --> 00:24:12.510 thing to go across in a controlled fashion. 00:24:12.510 --> 00:24:14.960 The third form is electrical potential. 00:24:22.160 --> 00:24:36.340 Again, the membrane actually acts as an insulator, and all 00:24:36.340 --> 00:24:52.540 cells, if this is the inside, and this is the outside, 00:24:52.540 --> 00:24:59.310 there's a gradient of hydrogen ions, so there are more 00:24:59.310 --> 00:25:03.810 hydrogen ions outside the cell than there are inside. 00:25:03.810 --> 00:25:09.450 So it creates an electrical potential. 00:25:09.450 --> 00:25:14.740 And these can't cross the membrane unless, guess what? 00:25:14.740 --> 00:25:17.560 There's a protein in the membrane that's able to permit 00:25:17.560 --> 00:25:20.190 their passage under controlled circumstances. 00:25:20.190 --> 00:25:24.760 So there's basically three different forms of energy that 00:25:24.760 --> 00:25:26.750 can be interconverted. 00:25:26.750 --> 00:25:29.660 And Peter Mitchell's great insight, which I will say was 00:25:29.660 --> 00:25:35.730 not intuitive for many people, was the combination, so the 00:25:35.730 --> 00:25:53.530 combo of this proton concentration gradient plus 00:25:53.530 --> 00:26:07.580 the electrical potential, could be used to drive the 00:26:07.580 --> 00:26:11.920 synthesis of ATP. 00:26:16.930 --> 00:26:19.190 And let me just say a couple of words. 00:26:19.190 --> 00:26:22.805 Because this may feel, how could this be? 00:26:22.805 --> 00:26:24.500 Could you really have energy? 00:26:24.500 --> 00:26:33.242 Well the potential across a cell is about 70 millivolts. 00:26:33.242 --> 00:26:35.690 May not seem all that much. 00:26:35.690 --> 00:26:42.720 But remember the membrane is about three nanometers thick. 00:26:42.720 --> 00:26:50.340 So that's about 200,000 volts per centimeter. 00:26:50.340 --> 00:26:55.620 High tension wires are 200,000 volts per mile or something. 00:26:55.620 --> 00:26:57.880 There's a lot of power in there. 00:26:57.880 --> 00:27:01.500 And furthermore, let's see if I can bring this up. 00:27:01.500 --> 00:27:04.060 I've been showing you this little 00:27:04.060 --> 00:27:05.380 movie a couple of times. 00:27:05.380 --> 00:27:08.040 The bacteria with these little nanomotors are spinning those 00:27:08.040 --> 00:27:11.900 flagella, and we saw how there's this machinery that's 00:27:11.900 --> 00:27:12.840 a little nanomotor. 00:27:12.840 --> 00:27:15.150 You know how it's powered? 00:27:15.150 --> 00:27:18.850 It's powered by the proton gradient. 00:27:18.850 --> 00:27:22.880 A proton trickles its way through this apparatus from 00:27:22.880 --> 00:27:24.460 the outside to the inside. 00:27:24.460 --> 00:27:26.010 It's coming down the gradient. 00:27:26.010 --> 00:27:28.600 That's the source of the power. 00:27:28.600 --> 00:27:31.310 And as I showed you, it's a pretty powerful motor. 00:27:31.310 --> 00:27:34.990 You can basically glue the propeller to a slide and it 00:27:34.990 --> 00:27:38.010 can twirl the bacteria all around. 00:27:38.010 --> 00:27:44.040 In fact, one of my favorite demos is, years ago people 00:27:44.040 --> 00:27:47.040 took a bacterium, and they managed to pop it open. 00:27:47.040 --> 00:27:53.280 So all the cytoplasm, all of the stuff on the inside came 00:27:53.280 --> 00:27:57.130 out of the cell, and you just got buffer on the inside. 00:27:57.130 --> 00:27:59.530 But it had these flagella. 00:27:59.530 --> 00:28:06.950 So you had just shells of bacteria with nothing really 00:28:06.950 --> 00:28:07.990 inside them. 00:28:07.990 --> 00:28:12.850 But, if you add a drop of acid to this media, now you've 00:28:12.850 --> 00:28:18.500 created a proton gradient with more protons on the outside 00:28:18.500 --> 00:28:20.970 than are on the inside, and guess what happens? 00:28:20.970 --> 00:28:24.540 The flagella motor starts working, and the bacteria 00:28:24.540 --> 00:28:27.710 start swimming, even though all the air, talk about dead 00:28:27.710 --> 00:28:29.640 man walking or something like that. 00:28:29.640 --> 00:28:35.080 It gives you an idea of the power that's in this 00:28:35.080 --> 00:28:40.720 combination of the proton gradient and 00:28:40.720 --> 00:28:42.210 the electric potential. 00:28:42.210 --> 00:28:49.970 The combination of this is often referred to as the 00:28:49.970 --> 00:28:51.230 proton motive force. 00:29:00.040 --> 00:29:02.450 So here's the principle of how the cell is 00:29:02.450 --> 00:29:04.220 able to exploit that. 00:29:04.220 --> 00:29:08.800 And this is what underlies respiration. 00:29:08.800 --> 00:29:10.050 There are two stages. 00:29:13.070 --> 00:29:19.810 Stage one, there's a membrane with some kind of membrane 00:29:19.810 --> 00:29:27.510 protein in it, which is actually a protein, functions 00:29:27.510 --> 00:29:31.300 as a proton pump. 00:29:31.300 --> 00:29:36.310 So it's a protein that's designed to be embedded into a 00:29:36.310 --> 00:29:39.640 membrane and to work there. 00:29:39.640 --> 00:29:43.550 This part here is the membrane itself. 00:29:43.550 --> 00:29:52.340 The proton gets transported from the inside to the outside 00:29:52.340 --> 00:29:57.350 when energy is put into this proton pump. 00:29:57.350 --> 00:30:03.160 So in response to some energy producing event, the cell 00:30:03.160 --> 00:30:08.790 pumps protons from its inside to its outside, and this then 00:30:08.790 --> 00:30:11.415 establishes the proton gradient. 00:30:22.160 --> 00:30:31.590 The second phase, then, is to take advantage of that proton 00:30:31.590 --> 00:30:37.160 gradient, and there's a different protein embedded in 00:30:37.160 --> 00:30:38.676 the membrane. 00:30:38.676 --> 00:30:43.890 It's known as an ATP synthase. 00:30:43.890 --> 00:30:51.550 And it permits a proton to come down the gradient, which 00:30:51.550 --> 00:30:52.880 you would want to do. 00:30:52.880 --> 00:30:54.830 But if that's all that happened, all you'd do is 00:30:54.830 --> 00:30:56.960 you'd just dissipate your gradient. 00:30:56.960 --> 00:31:02.580 So the key here is that this proton is only allowed to come 00:31:02.580 --> 00:31:06.530 down the gradient to the energetically more favorable 00:31:06.530 --> 00:31:12.750 side if ADP and inorganic phosphate are bound to this 00:31:12.750 --> 00:31:14.380 ATP synthase. 00:31:14.380 --> 00:31:18.340 And the dropping of the proton down the gradient's passage 00:31:18.340 --> 00:31:22.900 through this ATP synthase, which is an energy favorable 00:31:22.900 --> 00:31:26.400 reaction, drives the synthesis of ATP. 00:31:29.190 --> 00:31:31.850 So much energy is basically given off with this, you can 00:31:31.850 --> 00:31:35.970 make an ATP and the thing will still go. 00:31:35.970 --> 00:31:41.910 Now interestingly, this ATP synthase, which really lies at 00:31:41.910 --> 00:31:47.930 the heart of our energetics for how we function as human 00:31:47.930 --> 00:31:57.990 beings, is derived from it's crystal structure. 00:31:57.990 --> 00:32:01.600 But in fact, evolutionarily, it's related to 00:32:01.600 --> 00:32:04.360 that flagella motor. 00:32:04.360 --> 00:32:08.670 And as that proton comes down the gradient, or actually this 00:32:08.670 --> 00:32:11.470 is presented upside down, so there's the outside as it goes 00:32:11.470 --> 00:32:17.070 through in this direction, the ATP synthase, which is known 00:32:17.070 --> 00:32:29.990 as the F1F0 ATP synthase rotates. 00:32:29.990 --> 00:32:33.590 And probably this came first. 00:32:33.590 --> 00:32:35.390 It's a little hard in this one because you don't have the 00:32:35.390 --> 00:32:38.690 flagella, so what scientists have done is they've been able 00:32:38.690 --> 00:32:42.120 to attach something like an actin filament onto this F1 00:32:42.120 --> 00:32:47.130 ATP synthase, and show that as a proton 00:32:47.130 --> 00:32:49.650 passages the thing rotates. 00:32:49.650 --> 00:32:53.710 So in all likelihood what happened in evolution was this 00:32:53.710 --> 00:32:59.690 came first, and then later the machinery got duplicated and 00:32:59.690 --> 00:33:02.120 evolved to become a nanomotor. 00:33:02.120 --> 00:33:05.610 And as I told you the other day, that apparatus for the 00:33:05.610 --> 00:33:09.920 flagella motor got evolved again into becoming a little 00:33:09.920 --> 00:33:17.050 syringe that bacteria like ursinia are able to use to 00:33:17.050 --> 00:33:20.240 pump or to squeeze proteins or squirt proteins from inside 00:33:20.240 --> 00:33:25.780 them into inside of a mammalian cell. 00:33:25.780 --> 00:33:28.080 OK, well. 00:33:28.080 --> 00:33:32.460 Thanks to this work by Peter Mitchell then, we can now 00:33:32.460 --> 00:33:37.840 understand how cells were able to take advantage of that 00:33:37.840 --> 00:33:40.805 energy that was in the NADH. 00:33:44.680 --> 00:33:51.230 So this process is known as respiration. 00:33:51.230 --> 00:33:58.680 And basically it's taking the 2NADH. 00:34:02.060 --> 00:34:04.060 I'm supposed to see the physical therapist today, so I 00:34:04.060 --> 00:34:08.110 hope we're going to begin to make progress to lecturing on 00:34:08.110 --> 00:34:10.460 two feet sooner or later. 00:34:10.460 --> 00:34:19.570 Plus 2NAD+ plus two water. 00:34:19.570 --> 00:34:25.389 So as I said earlier, NADH and protons, 00:34:25.389 --> 00:34:27.909 it's basically hydrogen. 00:34:27.909 --> 00:34:32.090 It's the equivalent of having hydrogen gas and adding 00:34:32.090 --> 00:34:36.830 oxygen, and we're burning the hydrogen gas down to water. 00:34:36.830 --> 00:34:39.429 So there's a lot to yield water. 00:34:39.429 --> 00:34:42.560 So there's a lot of energy potentially can be given off. 00:34:42.560 --> 00:34:47.630 That's the 50 kcals per mole. 00:34:47.630 --> 00:34:51.730 Now if you recall when we talked about thermodynamics, 00:34:51.730 --> 00:35:08.660 so the NADH is up here, by the time we get down to the 2NAD+ 00:35:08.660 --> 00:35:11.830 plus the water, the two waters, 00:35:11.830 --> 00:35:13.960 energetically we're down here. 00:35:13.960 --> 00:35:17.930 And this is about a free energy changed of about 50 00:35:17.930 --> 00:35:20.990 kcals per mole. 00:35:20.990 --> 00:35:25.610 In physiological terms, that's a huge amount of energy. 00:35:25.610 --> 00:35:30.250 And I think some of the textbooks compare it to 00:35:30.250 --> 00:35:33.910 letting a stick of dynamite off inside of a cell. 00:35:33.910 --> 00:35:37.650 So it's really more than biology figured out how to 00:35:37.650 --> 00:35:39.780 handle this in a single step. 00:35:39.780 --> 00:35:41.800 But do you remember that important principle about a 00:35:41.800 --> 00:35:44.450 thermodynamic property, when I had the little 00:35:44.450 --> 00:35:45.410 picture of the skier? 00:35:45.410 --> 00:35:48.730 It doesn't matter which pathway you take. 00:35:48.730 --> 00:35:51.520 You get the same amount of energy released whether you go 00:35:51.520 --> 00:35:53.790 down the black diamond slope or you go 00:35:53.790 --> 00:35:55.420 down the bunny slope. 00:35:55.420 --> 00:36:01.750 So in fact, the way biology has learned, life has learned 00:36:01.750 --> 00:36:04.960 to control this amount of energy is basically taking the 00:36:04.960 --> 00:36:06.290 bunny slope. 00:36:06.290 --> 00:36:14.350 And so the energy drop occurs in a series of stages, where 00:36:14.350 --> 00:36:17.720 you have the transfer of two electrons to a lower state 00:36:17.720 --> 00:36:21.760 intermediate, transfer of two electrons to another one, 00:36:21.760 --> 00:36:25.400 transfer of two electrons to another one. 00:36:25.400 --> 00:36:28.560 And where this connects with the stuff that I just told 00:36:28.560 --> 00:36:32.800 you, is as these two electrons are coming down, what's 00:36:32.800 --> 00:36:38.740 happening is a proton is being pumped from the 00:36:38.740 --> 00:36:41.250 inside to the outside. 00:36:41.250 --> 00:36:47.750 As it moves to the next lower energy state, another proton 00:36:47.750 --> 00:36:51.860 gets pumped from the inside, the outside. 00:36:51.860 --> 00:36:53.590 And the same thing happens here. 00:36:58.300 --> 00:37:08.790 So at the end, you get the two hydrogens plus the half of an 00:37:08.790 --> 00:37:10.680 oxygen and we get a water molecule 00:37:10.680 --> 00:37:13.010 from these two electrons. 00:37:13.010 --> 00:37:18.890 But what's happened is these three protons have changed 00:37:18.890 --> 00:37:20.820 from inside to outside. 00:37:20.820 --> 00:37:26.500 That enables the cell to make three ATPs. 00:37:26.500 --> 00:37:30.740 So now instead of throwing away all that energy, losing 00:37:30.740 --> 00:37:33.560 the NADH as in the fermentations, the cell is 00:37:33.560 --> 00:37:37.940 extracting energy out of it by taking advantage of this 00:37:37.940 --> 00:37:42.440 principle of the proton gradient. 00:37:42.440 --> 00:37:45.720 So the game changes if you're this 00:37:45.720 --> 00:37:48.530 evolutionary designer or something. 00:37:48.530 --> 00:37:52.390 If you were trying to design life from first principles 00:37:52.390 --> 00:37:53.950 now, you could take advantage of this. 00:37:53.950 --> 00:37:56.350 Well of course it doesn't happen that way. 00:37:56.350 --> 00:38:00.620 Experiments happen all the time in nature and something 00:38:00.620 --> 00:38:02.940 happens and sometimes it's very efficient, 00:38:02.940 --> 00:38:03.860 sometimes it isn't. 00:38:03.860 --> 00:38:06.160 But if it's there first it gets going. 00:38:06.160 --> 00:38:12.440 In this case, the need now, or the opportunity was that if an 00:38:12.440 --> 00:38:20.505 organism could get more NADH out of that original molecule 00:38:20.505 --> 00:38:24.400 of glucose, it could make more energy than somebody else. 00:38:24.400 --> 00:38:29.950 And so the ultimate way to take a molecule of glucose is 00:38:29.950 --> 00:38:32.560 if you burn it with, oxygen you end up with six carbon 00:38:32.560 --> 00:38:33.990 dioxides and water. 00:38:33.990 --> 00:38:35.410 You burn it all away. 00:38:35.410 --> 00:38:39.590 So there's a system that, in essence, does that. 00:38:39.590 --> 00:38:43.130 It's known as the citric acid cycle. 00:38:47.230 --> 00:38:55.080 So you have the pyruvate that comes from glycolysis. 00:38:55.080 --> 00:39:00.680 And the way it's processed is first, one of the carboxyl 00:39:00.680 --> 00:39:03.620 group on the pyruvate is released, and 00:39:03.620 --> 00:39:05.480 this produces acetyl. 00:39:13.860 --> 00:39:15.610 You can look to see what CoA is. 00:39:15.610 --> 00:39:16.900 At the moment, it doesn't matter. 00:39:16.900 --> 00:39:20.830 What does matter is this is a 3-carbon compound. 00:39:20.830 --> 00:39:24.700 Acetyl, as you probably know, is a two-carbon compound. 00:39:24.700 --> 00:39:27.790 And when you look in your textbooks at the citric acid 00:39:27.790 --> 00:39:33.990 cycle, you'll see this very confusing circle with lots of 00:39:33.990 --> 00:39:38.360 compounds and enzymes and stuff. 00:39:38.360 --> 00:39:40.850 But I want you just keep your eye on the ball here. 00:39:40.850 --> 00:39:45.750 If you'll notice, the compound over here is in the cycle, is 00:39:45.750 --> 00:39:47.390 four carbons. 00:39:47.390 --> 00:39:50.480 And what happens is this 2-carbon compound that was 00:39:50.480 --> 00:39:55.170 derived from pyruvate gets added to this to give a 00:39:55.170 --> 00:39:57.080 6-carbon compound. 00:39:57.080 --> 00:40:01.690 And then that gets converted to a 5-carbon compound with a 00:40:01.690 --> 00:40:04.980 molecule of CO2 being given off. 00:40:04.980 --> 00:40:10.120 That in turn gets converted to a 4-carbon compound with 00:40:10.120 --> 00:40:13.480 another molecule of CO2 given off. 00:40:13.480 --> 00:40:16.400 And then there's some molecular gymnastics here that 00:40:16.400 --> 00:40:20.620 change the nature of the four carbon compound a bit so you 00:40:20.620 --> 00:40:22.940 can get back into the cycle. 00:40:22.940 --> 00:40:27.970 But look what's happened to those three carbons that were 00:40:27.970 --> 00:40:29.280 in the pyruvate. 00:40:29.280 --> 00:40:32.850 There's one of them, there's the other one, 00:40:32.850 --> 00:40:34.490 there's the other one. 00:40:34.490 --> 00:40:40.250 So this citric acid cycle produces, it actually makes 00:40:40.250 --> 00:40:45.840 some ATP, but it makes quite a bit of NADH. 00:40:45.840 --> 00:40:58.000 And it also makes another, one more reduced electron carrier. 00:40:58.000 --> 00:41:01.920 It's not NADH, it's another one that's used in the cell. 00:41:01.920 --> 00:41:07.160 But anyway, the cell is then able to take all of this NADH 00:41:07.160 --> 00:41:12.250 and this electron carrier plus these to give you, what I'd 00:41:12.250 --> 00:41:16.430 said, the net yield you get from respiration. 00:41:16.430 --> 00:41:25.330 36 ATPs from a single molecule of glucose. 00:41:25.330 --> 00:41:29.205 So sort of quite remarkable to some extent. 00:41:29.205 --> 00:41:32.960 We're looking at evolution, through, if you will, almost 00:41:32.960 --> 00:41:36.770 like looking at biochemical fossils and then when 00:41:36.770 --> 00:41:41.060 something works, it's a living fossil, we still 00:41:41.060 --> 00:41:43.550 find it in our cells.