1 00:00:00,000 --> 00:00:04,000 Just very quickly, I mentioned to you the other day 2 00:00:04,000 --> 00:00:08,000 that article about Romney making a policy statement about embryonic 3 00:00:08,000 --> 00:00:13,000 stem cell research and said here were sort of examples of issues we 4 00:00:13,000 --> 00:00:17,000 were thinking about in this course that would come into your ordinary 5 00:00:17,000 --> 00:00:22,000 life. Here's on today's Globe there's, ìKennedy rips Romney over 6 00:00:22,000 --> 00:00:26,000 stem cell policy. This debate is continuing. 7 00:00:26,000 --> 00:00:31,000 It's on the front page of today's Boston Globe. 8 00:00:31,000 --> 00:00:34,000 We have two people neither of whom probably have the background in 9 00:00:34,000 --> 00:00:38,000 biology, that you guys are going to have by the time you finish this 10 00:00:38,000 --> 00:00:41,000 course, having to grapple with these very serious scientific issues that 11 00:00:41,000 --> 00:00:45,000 have all kinds of implications. Here's another thing. This was in 12 00:00:45,000 --> 00:00:49,000 yesterday's newspaper. This was on, I think, the second 13 00:00:49,000 --> 00:00:52,000 page of the Boston Globe. It's about a curator over at 14 00:00:52,000 --> 00:00:56,000 Harvard at The Museum of Comparative Zoology that got all these different 15 00:00:56,000 --> 00:01:00,000 birds, some of whom are extinct and something. 16 00:01:00,000 --> 00:01:03,000 And now the curator, instead of just having them there as 17 00:01:03,000 --> 00:01:07,000 a sort of collection and that's all it is, is getting DNA out of these 18 00:01:07,000 --> 00:01:10,000 specimens and working back and looking at gaining new insights from 19 00:01:10,000 --> 00:01:14,000 animals that aren't found on this planet anymore. 20 00:01:14,000 --> 00:01:17,000 And this was one was from, I didn't want to show you this on 21 00:01:17,000 --> 00:01:21,000 Valentine's Day particularly, but this is critical. I don't 22 00:01:21,000 --> 00:01:25,000 subscribe to The New York Times but for some reason one appeared on my 23 00:01:25,000 --> 00:01:29,000 porch on Saturday morning. I opened up the first page, 24 00:01:29,000 --> 00:01:33,000 ìRare and aggressive HIV reported in New York.î I'll be talking to you 25 00:01:33,000 --> 00:01:37,000 about the HIV-1 virus which is the virus that ends up destroying some 26 00:01:37,000 --> 00:01:42,000 of your major defender cells in the immune system. 27 00:01:42,000 --> 00:01:46,000 And then people die of AIDS which is sort of all the other stuff that 28 00:01:46,000 --> 00:01:51,000 then happens. And it was terribly scary when it first showed up. 29 00:01:51,000 --> 00:01:55,000 There was no treatment for it. It's like that in most of the Third 30 00:01:55,000 --> 00:01:59,000 World and people die if they get it. I lost a close friend to it after 31 00:01:59,000 --> 00:02:03,000 it first came up. Then they came up with these various 32 00:02:03,000 --> 00:02:07,000 cocktails of different inhibitors. And the idea was instead of just 33 00:02:07,000 --> 00:02:10,000 having a single drug target where it would be easy for the virus to 34 00:02:10,000 --> 00:02:14,000 mutate and, as I'll tell you, the HIV virus mutates like crazy. 35 00:02:14,000 --> 00:02:17,000 Instead they'd use multiple targets and that's why they do it. 36 00:02:17,000 --> 00:02:21,000 And sort of then the chances of it getting a mutation, 37 00:02:21,000 --> 00:02:24,000 you multiply all these low probabilities of getting a mutation 38 00:02:24,000 --> 00:02:28,000 and would hope it would never happen. It's happened. 39 00:02:28,000 --> 00:02:31,000 There in New York is a version of HIV-1 that is resistant to 40 00:02:31,000 --> 00:02:35,000 everything. If that spreads, which it probably will, it will be 41 00:02:35,000 --> 00:02:38,000 just like what happened with all the bacteria and antibiotics. 42 00:02:38,000 --> 00:02:42,000 It works for a little while, natural mutation and natural 43 00:02:42,000 --> 00:02:45,000 selection, something will happen, the organism or the virus will 44 00:02:45,000 --> 00:02:49,000 change so that it's no longer resistant. So, 45 00:02:49,000 --> 00:02:52,000 you know, for the moment, as always, you know, make sure you 46 00:02:52,000 --> 00:02:56,000 never expose yourself to AIDS. This would take us back just like 47 00:02:56,000 --> 00:03:00,000 to before any of these things were inhibited. 48 00:03:00,000 --> 00:03:03,000 And the same thing goes right now. There are strains of mycobacterium 49 00:03:03,000 --> 00:03:06,000 tuberculosis that are resistant to every known antibiotic. 50 00:03:06,000 --> 00:03:10,000 If you get tuberculosis with that thing it's like you were living 200 51 00:03:10,000 --> 00:03:13,000 years ago. They cannot do anything for you. They'd put you in a 52 00:03:13,000 --> 00:03:17,000 sanatorium but you cannot be cured. This is worse. You know, this is 53 00:03:17,000 --> 00:03:20,000 awful. This one kills you. So just be careful. This is just 54 00:03:20,000 --> 00:03:24,000 the tip, but what always happens when you have antibiotics or drug 55 00:03:24,000 --> 00:03:27,000 treatments eventually natural selection mutation will probably 56 00:03:27,000 --> 00:03:31,000 produce a variant. And this looks like it is happening. 57 00:03:31,000 --> 00:03:34,000 I don't know, I hope not, but I'm afraid this won't be the end 58 00:03:34,000 --> 00:03:38,000 of this one. OK. So some of you, I've read some of 59 00:03:38,000 --> 00:03:42,000 the comments. I'm not terribly surprised that some of you think why 60 00:03:42,000 --> 00:03:46,000 is he telling me all this. God, there was a lot of work for 61 00:03:46,000 --> 00:03:49,000 tremendously confusing, there were all these chemical 62 00:03:49,000 --> 00:03:53,000 structures, I didn't really get it all. It seemed that somehow out of 63 00:03:53,000 --> 00:03:57,000 this the cell got to two molecules of ATP. Why are we wasting all the 64 00:03:57,000 --> 00:04:01,000 time? Why am I wasting all this time telling you about how to get 65 00:04:01,000 --> 00:04:06,000 two molecules of ATP? This is basically what we were doing. 66 00:04:06,000 --> 00:04:12,000 We were looking at glucose, which is C6H12O6, going through this 67 00:04:12,000 --> 00:04:19,000 process which pathway is of ten enzymatic steps. 68 00:04:19,000 --> 00:04:26,000 And what ends up coming out at the other end is two pyruvates. 69 00:04:26,000 --> 00:04:32,000 And the net yield of that is two ATP plus two molecules of this weird 70 00:04:32,000 --> 00:04:37,000 thing called NADH. Now, I think most of you, 71 00:04:37,000 --> 00:04:40,000 although a few of you still don't seem to have gotten it, 72 00:04:40,000 --> 00:04:44,000 the reason you need to make ATP out of ADP and inorganic phosphate is it 73 00:04:44,000 --> 00:04:47,000 stores energy. It's, you know, 74 00:04:47,000 --> 00:04:50,000 like having electricity in your house or batteries in your 75 00:04:50,000 --> 00:04:54,000 flashlight or in your iPod or whatever it is. 76 00:04:54,000 --> 00:04:57,000 You've got to have energy in order to do stuff. And cells need it. 77 00:04:57,000 --> 00:05:01,000 And the major sort of energy currency they use is ATP. 78 00:05:01,000 --> 00:05:04,000 And they put most of it by squeezing together a phosphate up to an ATP 79 00:05:04,000 --> 00:05:08,000 making this bond that's stable, unless there's an enzyme that'll 80 00:05:08,000 --> 00:05:12,000 break it open, but there's energy stored in it. 81 00:05:12,000 --> 00:05:15,000 And they'll drop downhill and you can use that energy to drive other 82 00:05:15,000 --> 00:05:19,000 reactions. So for life to happen it had to somehow figure out how to get 83 00:05:19,000 --> 00:05:23,000 energy in a form that it could use to do all the biosynthesis that's 84 00:05:23,000 --> 00:05:27,000 necessary for life to go on. NADH, I showed you the molecule, 85 00:05:27,000 --> 00:05:30,000 you got a structure. The cell makes some number of 86 00:05:30,000 --> 00:05:34,000 molecules of that. It's around. And what happened, 87 00:05:34,000 --> 00:05:38,000 it comes in, it helped some of these processes go. It ends up, 88 00:05:38,000 --> 00:05:42,000 it's sort of like a banking system for electrons. 89 00:05:42,000 --> 00:05:45,000 If it gets NADH it's got electrons stuck on it. There are only a 90 00:05:45,000 --> 00:05:49,000 limited number of molecules of NADH in a cell, so if you make all the 91 00:05:49,000 --> 00:05:53,000 NAD molecules into NADH then you have nowhere else to put electrons 92 00:05:53,000 --> 00:05:57,000 and everything comes to a grinding halt. Because I was talking about 93 00:05:57,000 --> 00:06:01,000 redox reactions. If you're taking electrons off 94 00:06:01,000 --> 00:06:05,000 somewhere they've got to go somewhere else. 95 00:06:05,000 --> 00:06:09,000 And the cell uses NADH as kind of almost a currency for passing 96 00:06:09,000 --> 00:06:13,000 electrons around in just the same way it uses ATP as a currency for 97 00:06:13,000 --> 00:06:18,000 carrying energy around from one form or another. Now, 98 00:06:18,000 --> 00:06:22,000 the problem was and why this was so important is this apparently, 99 00:06:22,000 --> 00:06:26,000 as I said, we're basically looking at an evolutionary fossil of some 100 00:06:26,000 --> 00:06:30,000 sort because every organism on earth uses, or virtually every organism on 101 00:06:30,000 --> 00:06:35,000 earth uses glycolysis. It's in our cytoplasm of our cells. 102 00:06:35,000 --> 00:06:39,000 It's in the cytoplasm of bacteria. It's in the cytoplasm of archaea. 103 00:06:39,000 --> 00:06:44,000 It apparently evolved so long ago that every form of life on earth, 104 00:06:44,000 --> 00:06:48,000 you know, depends on this, anything that can metabolize a sugar molecule 105 00:06:48,000 --> 00:06:53,000 pretty much. And so it looks complicated. It is. 106 00:06:53,000 --> 00:06:57,000 Maybe you could have designed, I'd like to tell you that it's two 107 00:06:57,000 --> 00:07:02,000 reactions and you make two ATPs and it's all simple. 108 00:07:02,000 --> 00:07:04,000 Life isn't often simple. It's often very complicated. 109 00:07:04,000 --> 00:07:07,000 It wasn't done by a team of design engineers in Building 10 designing 110 00:07:07,000 --> 00:07:10,000 it. It was done by something that happened with random selection. 111 00:07:10,000 --> 00:07:13,000 And when something finally worked apparently it got fixed in evolution 112 00:07:13,000 --> 00:07:15,000 even if it looks clunky. I think the other way to look at it 113 00:07:15,000 --> 00:07:18,000 is to say isn't that, I feel a sort of sense of wonder at 114 00:07:18,000 --> 00:07:21,000 this point, and granted I've had a lot of years working in the field, 115 00:07:21,000 --> 00:07:24,000 that something that complicated could work out and take this 116 00:07:24,000 --> 00:07:27,000 molecule of glucose and make it into two molecules of ATP that can then 117 00:07:27,000 --> 00:07:30,000 be used to allow the cell to do synthesize. 118 00:07:30,000 --> 00:07:35,000 As in I'll show you, there's a better way you could use 119 00:07:35,000 --> 00:07:40,000 this much more efficiently, 18 times more efficiently if we do 120 00:07:40,000 --> 00:07:45,000 respiration, which we'll talk about today, but that needs oxygen. 121 00:07:45,000 --> 00:07:51,000 And when life started 3.8 billion years ago or so there was no oxygen 122 00:07:51,000 --> 00:07:56,000 so it wasn't an option. The initial organisms had to make 123 00:07:56,000 --> 00:08:02,000 due with what they found and this is what developed. 124 00:08:02,000 --> 00:08:05,000 So ATP can go off and be used for lots of things. 125 00:08:05,000 --> 00:08:09,000 The problem, if there's no oxygen around, as I said there are a 126 00:08:09,000 --> 00:08:12,000 limited number of molecules of ATP. So to make that glycolysis go 127 00:08:12,000 --> 00:08:16,000 you're taking some electrons off and doing the oxidation steps, 128 00:08:16,000 --> 00:08:19,000 you're reducing NADH. And if you want that to go as a cycle you've 129 00:08:19,000 --> 00:08:23,000 got to somehow regenerate the NAD+ so you can go through another cycle 130 00:08:23,000 --> 00:08:27,000 and make more ATP. So there were two variants that I 131 00:08:27,000 --> 00:08:31,000 told you about. If you don't have oxygen around, 132 00:08:31,000 --> 00:08:37,000 one of these variants yields to lactate, and so the net yield of 133 00:08:37,000 --> 00:08:42,000 this is two ATP. Because what happens in this part 134 00:08:42,000 --> 00:08:48,000 is it uses up two molecules of NADH changing the two molecules of 135 00:08:48,000 --> 00:08:53,000 pyruvate to two molecules of lactate, so that gets rid of this, 136 00:08:53,000 --> 00:08:59,000 and the net yield of the whole thing is two molecules of ATP. 137 00:08:59,000 --> 00:09:03,000 The other way to do it is the way yeast does it. 138 00:09:03,000 --> 00:09:07,000 It makes two molecules of CO2 plus two molecules of ethanol. 139 00:09:07,000 --> 00:09:11,000 And I think when I wrote that on the board I showed you that there 140 00:09:11,000 --> 00:09:15,000 was acetaldehyde. We had two molecules of that going 141 00:09:15,000 --> 00:09:19,000 to two molecules of ethanol. And I think I forgot to write on 142 00:09:19,000 --> 00:09:23,000 the board that it used up two molecules of NADH because that's 143 00:09:23,000 --> 00:09:27,000 important. So that's what this whole thing is about. 144 00:09:27,000 --> 00:09:31,000 It achieves the same thing. And the net yield of this one is two 145 00:09:31,000 --> 00:09:35,000 ATPs as well. So I'm going to talk about photosynthesis at the latter 146 00:09:35,000 --> 00:09:39,000 part of the lecture, but if you maybe recall the initial 147 00:09:39,000 --> 00:09:43,000 sort of thing I called ìrelease oneî came up here about 3. 148 00:09:43,000 --> 00:09:47,000 billion years ago. Later on an improved version came along that 149 00:09:47,000 --> 00:09:51,000 started to generate oxygen. Now, it took billions of years for 150 00:09:51,000 --> 00:09:55,000 us to get to our present level of oxygen, but when oxygen became 151 00:09:55,000 --> 00:09:59,000 available in the atmosphere then there was a new option 152 00:09:59,000 --> 00:10:02,000 for making energy. And that was instead of taking this 153 00:10:02,000 --> 00:10:06,000 NADH and, remember, there are about 50 kcals per mole of 154 00:10:06,000 --> 00:10:10,000 energy in there, it's just being thrown away. 155 00:10:10,000 --> 00:10:13,000 It's not being used at all. But if there's oxygen around there's a new 156 00:10:13,000 --> 00:10:17,000 and better system, which you know as respiration, 157 00:10:17,000 --> 00:10:20,000 it's a word you've heard. You know we breathe oxygen. 158 00:10:20,000 --> 00:10:24,000 We need to breathe oxygen because we're using the oxygen to make 159 00:10:24,000 --> 00:10:28,000 energy. And I'll tell you again. You don't have to know all the 160 00:10:28,000 --> 00:10:33,000 details. But there's a biochemical cycle 161 00:10:33,000 --> 00:10:39,000 called a citric acid cycle. And it goes together with another 162 00:10:39,000 --> 00:10:46,000 process that's known as oxidative phosphorylation. 163 00:10:46,000 --> 00:10:55,000 And so this is if we have oxygen 164 00:10:55,000 --> 00:11:00,000 what we end up with at the end of the day instead of these products is 165 00:11:00,000 --> 00:11:06,000 six molecules of CO2 plus six molecules of water. 166 00:11:06,000 --> 00:11:14,000 But more importantly 36 molecules of ATP. So respiration is tremendously 167 00:11:14,000 --> 00:11:23,000 better at capturing energy from a glucose molecule. 168 00:11:23,000 --> 00:11:31,000 But this, I'll show you, is a later arriving development in 169 00:11:31,000 --> 00:11:37,000 evolution. It had to be because it required 170 00:11:37,000 --> 00:11:40,000 oxygen in the atmosphere. And so even though we only get two 171 00:11:40,000 --> 00:11:43,000 molecules out of ATP. We all do it. And the other thing 172 00:11:43,000 --> 00:11:47,000 it generates is pyruvate. And, as you'll see, this process 173 00:11:47,000 --> 00:11:50,000 takes those pyruvates. That's the starting material for 174 00:11:50,000 --> 00:11:53,000 this part over here. OK? I hope this is making a bit 175 00:11:53,000 --> 00:11:56,000 more sense as to why you've got to keep your eye on the big picture or 176 00:11:56,000 --> 00:11:59,000 all you're going to see is a whole lot of structures and chemical 177 00:11:59,000 --> 00:12:03,000 transformations that don't make any sense. 178 00:12:03,000 --> 00:12:07,000 This is all about making ATP and energy. And, as we'll see later on, 179 00:12:07,000 --> 00:12:11,000 photosynthesis. In cases it can be something to do with making reducing 180 00:12:11,000 --> 00:12:15,000 power for biosynthesis. So in order to understand this, 181 00:12:15,000 --> 00:12:19,000 though, I have to introduce you to another way of thinking about energy. 182 00:12:19,000 --> 00:12:23,000 Some of you still seem to be struggling with the idea that you 183 00:12:23,000 --> 00:12:27,000 can store energy in a chemical bond, but I think from the comments the 184 00:12:27,000 --> 00:12:32,000 majority of you have that. If you're having trouble, 185 00:12:32,000 --> 00:12:37,000 ask your TA or look in sections and stuff. But the insight to how this 186 00:12:37,000 --> 00:12:42,000 part came, even though this process, what underlies this was invented 187 00:12:42,000 --> 00:12:47,000 about roughly 3. billion years ago, 188 00:12:47,000 --> 00:12:52,000 scientists didn't begin to even get a glimmer of how this worked until 189 00:12:52,000 --> 00:12:57,000 about 1961 when there was a scientist named Peter Mitchell who 190 00:12:57,000 --> 00:13:02,000 got a Nobel Prize for the insight that he had. 191 00:13:02,000 --> 00:13:07,000 And what he recognized was there were sort of three forms of energy 192 00:13:07,000 --> 00:13:13,000 that are interconvertable. The energy of a chemical bond. 193 00:13:13,000 --> 00:13:18,000 And I keep telling you this, that ATP, if we hydrolyze that it goes to 194 00:13:18,000 --> 00:13:24,000 ATP plus inorganic phosphate. The delta G prime zero is about 195 00:13:24,000 --> 00:13:29,000 minus 7 kilocalories per mole, but under physiological conditions 196 00:13:29,000 --> 00:13:35,000 it works out that each ATP gets you about 12 kilocalories per mole 197 00:13:35,000 --> 00:13:41,000 because life doesn't happen under these standard conditions. 198 00:13:41,000 --> 00:13:44,000 So that's one form of energy we've been talking a lot about. 199 00:13:44,000 --> 00:13:47,000 There's another form of energy, which you probably know intuitively, 200 00:13:47,000 --> 00:13:50,000 and that is if you have a high concentration of some compound on a 201 00:13:50,000 --> 00:13:53,000 side of a sort of impermeable barrier and a low concentration on 202 00:13:53,000 --> 00:13:56,000 the other side, high concentration of sugar, 203 00:13:56,000 --> 00:14:00,000 low concentration of sugar. If you give the system a chance it 204 00:14:00,000 --> 00:14:04,000 will come to equilibrium. So the concentrated stuff will flow 205 00:14:04,000 --> 00:14:09,000 downhill until you're concentrated on both sides. 206 00:14:09,000 --> 00:14:14,000 There's energy that you could get out of that. So there's energy 207 00:14:14,000 --> 00:14:18,000 basically stored in a concentration gradient. There's another form 208 00:14:18,000 --> 00:14:23,000 which should probably be familiar certainly to some of you, 209 00:14:23,000 --> 00:14:28,000 and that's the idea you can store energy in an electrical gradient. 210 00:14:28,000 --> 00:14:32,000 If you have some kind of impermeable barrier and we have a lot of charges 211 00:14:32,000 --> 00:14:36,000 on one side and less charges on the other side then there's a polarity, 212 00:14:36,000 --> 00:14:40,000 there's an electrical gradient, and that's a form of stored energy. 213 00:14:40,000 --> 00:14:44,000 Now, it happens that a membrane is impermeable, as I've told you, 214 00:14:44,000 --> 00:14:48,000 to most things. So you can get a high concentration of something on 215 00:14:48,000 --> 00:14:52,000 one side and a low concentration on the other and controlled by a 216 00:14:52,000 --> 00:14:56,000 protein imbedded in it, whether those ever get a chance to 217 00:14:56,000 --> 00:15:00,000 come across. The same idea applies for an electrical gradient. 218 00:15:00,000 --> 00:15:06,000 And in particular of interest to biology are hydrogen ions. 219 00:15:06,000 --> 00:15:12,000 So if we have a situation like this where we have more hydrogen ions on 220 00:15:12,000 --> 00:15:18,000 one side of a membrane than another then we have an electrical gradient, 221 00:15:18,000 --> 00:15:24,000 we have a polarity. And so this is the membrane. And so all 222 00:15:24,000 --> 00:15:35,000 cells then have -- 223 00:15:35,000 --> 00:15:39,000 And the way it works is this is the outside of the cell and this is the 224 00:15:39,000 --> 00:15:44,000 inside of the cell, so more pluses on the outside, 225 00:15:44,000 --> 00:15:49,000 hydrogen ions on the outside than the inside. And it's about 70 226 00:15:49,000 --> 00:15:54,000 millivolts. It may not sound that impressive, right? 227 00:15:54,000 --> 00:15:59,000 Another way to look at it, though, is the membrane is about 228 00:15:59,000 --> 00:16:04,000 three nanometers. So if you say, 229 00:16:04,000 --> 00:16:08,000 well, OK, what's the electrical gradient across that? 230 00:16:08,000 --> 00:16:12,000 It's about 200,000 volts per centimeter. And high tension lines 231 00:16:12,000 --> 00:16:17,000 are, you know, more like that per mile or something 232 00:16:17,000 --> 00:16:21,000 like that. So although it seems modest because the membranes, 233 00:16:21,000 --> 00:16:25,000 you've got have a very, very powerful electrical gradient in all 234 00:16:25,000 --> 00:16:30,000 cells. And so this is a form of energy. 235 00:16:30,000 --> 00:16:34,000 And I think this was proposed in 1961. Textbooks often say it was 236 00:16:34,000 --> 00:16:38,000 adopted in the early 70s. I went to a post-doc at Berkley in 237 00:16:38,000 --> 00:16:42,000 1975 and people were still having arguments about whether this really 238 00:16:42,000 --> 00:16:46,000 was the way, was this really something that was used in nature? 239 00:16:46,000 --> 00:16:50,000 It is. And I think one of the most dramatic demonstrations that a 240 00:16:50,000 --> 00:16:54,000 proton gradient can be a source of energy comes from this sort of thing. 241 00:16:54,000 --> 00:16:58,000 I'd showed you how this bacteria, these are E. coli swimming around 242 00:16:58,000 --> 00:17:02,000 with these rotary motors that spin 10,000 to 100,000 RPM driving 243 00:17:02,000 --> 00:17:05,000 those flagella. I showed you the picture of the 244 00:17:05,000 --> 00:17:08,000 inner membrane of E. coli. And I mentioned it has a 245 00:17:08,000 --> 00:17:11,000 double membrane. This is sort of a protective layer. 246 00:17:11,000 --> 00:17:14,000 You'll see, a little bit later in the lecture, some double membranes 247 00:17:14,000 --> 00:17:18,000 again. And here's the motor with the big propeller, 248 00:17:18,000 --> 00:17:21,000 the flagella sticking out from it. And the way this thing, and I 249 00:17:21,000 --> 00:17:24,000 showed you, I guess, this picture and then this textbook 250 00:17:24,000 --> 00:17:27,000 diagram. What drives this motor are protons 251 00:17:27,000 --> 00:17:31,000 flowing from the outside of the cell through here, through the proteins 252 00:17:31,000 --> 00:17:35,000 in here, through channels in them. And that's what provides the torque 253 00:17:35,000 --> 00:17:39,000 for the motor. That's what drives it. 254 00:17:39,000 --> 00:17:43,000 It's not ATP or anything. It's protons on the outside and 255 00:17:43,000 --> 00:17:47,000 inside. And there's a very dramatic demonstration that's sort of like 256 00:17:47,000 --> 00:17:51,000 Friday night horror films, [if you will?], where people found a 257 00:17:51,000 --> 00:17:55,000 way of popping an E. coli open so that all of the 258 00:17:55,000 --> 00:17:59,000 cytoplasm ran out and then it would reseal. 259 00:17:59,000 --> 00:18:02,000 So what you've got is sort of an E. coli that is just a shell, just the 260 00:18:02,000 --> 00:18:06,000 membranes and the proteins imbedded in it, and it just sits there at 261 00:18:06,000 --> 00:18:09,000 neutral pH. If you now acidify the medium what's happened is you've 262 00:18:09,000 --> 00:18:13,000 created a proton gradient because there are now more protons on the 263 00:18:13,000 --> 00:18:17,000 outside. And this is just the same picture I showed you before. 264 00:18:17,000 --> 00:18:20,000 But what happens if you do that experiment is the bacteria start 265 00:18:20,000 --> 00:18:24,000 swimming. They don't have any insides or anything but you have 266 00:18:24,000 --> 00:18:28,000 created artificially a proton gradient and it drives the motor and 267 00:18:28,000 --> 00:18:32,000 they start swimming. It's sort of like ìdead man walkingî 268 00:18:32,000 --> 00:18:38,000 or something at a bacteria level. I think it's a really dramatic 269 00:18:38,000 --> 00:18:43,000 demonstration of how you can use the energy of a proton gradient to make 270 00:18:43,000 --> 00:18:49,000 energy. So the principle that underlies how respiration works and 271 00:18:49,000 --> 00:18:54,000 photosynthesis works is that you take advantage of this combination 272 00:18:54,000 --> 00:19:00,000 of concentration and electrical gradient. 273 00:19:00,000 --> 00:19:10,000 And it's known as the chemi -- 274 00:19:10,000 --> 00:19:13,000 -- osmotic hypothesis. Because here you have an electrical 275 00:19:13,000 --> 00:19:17,000 gradient because of the charges. You also have a concentration 276 00:19:17,000 --> 00:19:20,000 gradient because you've got more hydrogen ions on this side. 277 00:19:20,000 --> 00:19:24,000 So you cannot really separate them. They're kind of coupled. But the 278 00:19:24,000 --> 00:19:28,000 idea is that life uses this proton gradient in order to make energy and 279 00:19:28,000 --> 00:19:32,000 do some of these energy transactions. 280 00:19:32,000 --> 00:19:35,000 So here's the sort of principle of how it's done. 281 00:19:35,000 --> 00:19:39,000 You have a membrane like this, then we have a protein, and now 282 00:19:39,000 --> 00:19:42,000 we're not going to see all the alpha helices and beta sheets. 283 00:19:42,000 --> 00:19:46,000 It'll be one of those things we talked about that spans the membrane. 284 00:19:46,000 --> 00:19:49,000 The membrane, as you might guess from what you 285 00:19:49,000 --> 00:19:53,000 know about it, is impermeable to hydrogen ions. 286 00:19:53,000 --> 00:19:57,000 So what this protein does that's imbedded in the membrane, 287 00:19:57,000 --> 00:20:00,000 it's a proton pump. And if you provide it with energy in 288 00:20:00,000 --> 00:20:04,000 some form what it does is it takes a hydrogen ion that's on the inside 289 00:20:04,000 --> 00:20:08,000 and it makes it into a hydrogen ion on the outside. 290 00:20:08,000 --> 00:20:11,000 There's no chemical transformation of the proton. 291 00:20:11,000 --> 00:20:15,000 It's just gone from one side of the membrane to the other. 292 00:20:15,000 --> 00:20:19,000 It's almost like recharging a battery or something if you want to 293 00:20:19,000 --> 00:20:23,000 think about it perhaps in that kind of way. 294 00:20:23,000 --> 00:20:32,000 And then the second stage is once the proton gradient is established 295 00:20:32,000 --> 00:20:42,000 and you have now many more H+s on the outside than on the inside then 296 00:20:42,000 --> 00:20:52,000 there is another protein that lets the proton flow. 297 00:20:52,000 --> 00:21:02,000 The proton flows down to gradient and therefore is able to come back 298 00:21:02,000 --> 00:21:09,000 inside the cell. Now, if that's all there was to it 299 00:21:09,000 --> 00:21:14,000 we wouldn't have achieved anything. We would have wasted energy, pump 300 00:21:14,000 --> 00:21:18,000 something out, pump something back. 301 00:21:18,000 --> 00:21:23,000 But this molecule has an interesting property, 302 00:21:23,000 --> 00:21:28,000 and that is the ability of the proton to flow down the gradient 303 00:21:28,000 --> 00:21:33,000 obligatorily requires ATP plus inorganic phosphate. 304 00:21:33,000 --> 00:21:37,000 And as the proton comes down this energy gradient there's enough 305 00:21:37,000 --> 00:21:41,000 energy given off by that the cell is able to capture it and use that 306 00:21:41,000 --> 00:21:45,000 energy to synthesize a molecule of ATP. Got it? So you produce some 307 00:21:45,000 --> 00:21:50,000 energy like from the light in photosynthesis that we'll see in a 308 00:21:50,000 --> 00:21:54,000 minute, other ways of doing it, and then get it outside. Once 309 00:21:54,000 --> 00:21:59,000 you've got the gradient now you can make ATP. 310 00:21:59,000 --> 00:22:04,000 And, actually, one of the completely remarkable 311 00:22:04,000 --> 00:22:10,000 discoveries of structural biology, this is known as an ATP synthase. 312 00:22:10,000 --> 00:22:15,000 It's a protein. It's an enzyme. 313 00:22:15,000 --> 00:22:21,000 These are the kinds of things we've been talking about. 314 00:22:21,000 --> 00:22:27,000 This is also a protein. You see all the different things 315 00:22:27,000 --> 00:22:31,000 proteins do. So this ATP synthase, 316 00:22:31,000 --> 00:22:35,000 which uses this energy of the proton gradient to make ATP, 317 00:22:35,000 --> 00:22:39,000 its structure has been worked out and at a level, 318 00:22:39,000 --> 00:22:43,000 here's part of the crystal structure. You can probably see some alpha 319 00:22:43,000 --> 00:22:47,000 helices beta sheets. Here's a textbook diagram of it 320 00:22:47,000 --> 00:22:51,000 showing, it's upside down I'm showing, but here's a proton on the 321 00:22:51,000 --> 00:22:55,000 inside flowing across. And what's remarkable is it turns 322 00:22:55,000 --> 00:22:59,000 out that this ATP synthase is structurally related to the protein 323 00:22:59,000 --> 00:23:03,000 that's at the heart of that rotary motor that drives the flagella. 324 00:23:03,000 --> 00:23:06,000 And, in fact, remember, I think I showed you where you could 325 00:23:06,000 --> 00:23:09,000 stick the flagella to a cover slip and the bacteria twirled around so 326 00:23:09,000 --> 00:23:12,000 you could actually see they were rotating? So people did sort of the 327 00:23:12,000 --> 00:23:15,000 equivalent thing, they managed to stick this ATP 328 00:23:15,000 --> 00:23:18,000 synthase. And you couldn't see that the top part was turning, 329 00:23:18,000 --> 00:23:21,000 but they used some tricky stuff we'll talk about with antibodies and 330 00:23:21,000 --> 00:23:24,000 there's something to attach a long filamentous molecule. 331 00:23:24,000 --> 00:23:27,000 It's the polymer of proton called actin. That's the same stuff we 332 00:23:27,000 --> 00:23:31,000 find in our muscles. And it made it long enough. 333 00:23:31,000 --> 00:23:35,000 You could see that when this thing was working that it was twirling 334 00:23:35,000 --> 00:23:40,000 around. And so evolution took this same basic sort of piece of protein 335 00:23:40,000 --> 00:23:44,000 machinery. And in one case it used it to capture the energy of a proton 336 00:23:44,000 --> 00:23:48,000 gradient and make ATP. And in another case it used it to 337 00:23:48,000 --> 00:23:53,000 drive this propeller, if you will. And here's sort of a 338 00:23:53,000 --> 00:23:57,000 simple diagram. So as the proton flows kind of what 339 00:23:57,000 --> 00:24:02,000 happens is the inner part of this thing sort of goes click, click. 340 00:24:02,000 --> 00:24:05,000 And every time it does it synthesizes an ATP. 341 00:24:05,000 --> 00:24:09,000 And it sort of takes that energy to push together the ADP and the 342 00:24:09,000 --> 00:24:12,000 inorganic phosphate overcoming that activation energy and getting them 343 00:24:12,000 --> 00:24:16,000 close enough that you're able to get that bond. It's a truly remarkable 344 00:24:16,000 --> 00:24:20,000 thing. This is a somewhat hard concept to grasp, 345 00:24:20,000 --> 00:24:23,000 I understand, but if you can understand this endocrine 346 00:24:23,000 --> 00:24:27,000 variability between energy in the form of a combined concentration of 347 00:24:27,000 --> 00:24:31,000 electrical gradient and ATP, and nature goes back and back and 348 00:24:31,000 --> 00:24:35,000 forth, it's absolutely fundamental to life. 349 00:24:35,000 --> 00:24:38,000 If we didn't have this stuff going on we couldn't do it. 350 00:24:38,000 --> 00:24:41,000 You know, as I say with glycolysis, I wish it was simpler, but this is 351 00:24:41,000 --> 00:24:44,000 the way nature did it, this is the way we are, 352 00:24:44,000 --> 00:24:47,000 and [what I know about?] biology this is how it goes. 353 00:24:47,000 --> 00:24:50,000 And I wouldn't be doing it justice if I didn't tell you some of the 354 00:24:50,000 --> 00:24:54,000 details. You're MIT students. You should be able to, I hope, 355 00:24:54,000 --> 00:24:57,000 some of how the world actually works at this kind of level. 356 00:24:57,000 --> 00:25:00,000 OK. Now, with that kind of background, I think 357 00:25:00,000 --> 00:25:04,000 we can kind of -- Pretty quickly I can help you begin 358 00:25:04,000 --> 00:25:08,000 to see what happens here. Now, remember the problem up there 359 00:25:08,000 --> 00:25:12,000 with glycolysis? It started when there was no oxygen, 360 00:25:12,000 --> 00:25:16,000 and therefore it generated these NADHs. They weren't any good. 361 00:25:16,000 --> 00:25:20,000 You had to just get rid of them so you took the pyruvate, 362 00:25:20,000 --> 00:25:24,000 organisms learned how to put them to make lactate or ethanol and carbon 363 00:25:24,000 --> 00:25:28,000 dioxide, but if there's oxygen around then there's another 364 00:25:28,000 --> 00:25:33,000 possibility. And that is you can combine these 365 00:25:33,000 --> 00:25:40,000 molecules with oxygen. So if we take two NADH plus two 366 00:25:40,000 --> 00:25:47,000 hydrogen ions plus a molecule of oxygen what we get is two waters. 367 00:25:47,000 --> 00:25:54,000 And I think I can show you what's going on there kind of simply. 368 00:25:54,000 --> 00:26:01,000 What NADH was, we got a pair of electrons here and a hydrogen ion. 369 00:26:01,000 --> 00:26:05,000 Well, if we took that, what's that? I think you'd 370 00:26:05,000 --> 00:26:10,000 recognize it as a molecule of hydrogen gas. So if you take NADH 371 00:26:10,000 --> 00:26:15,000 and oxygen, what the cell is really essentially doing, 372 00:26:15,000 --> 00:26:20,000 it's taking hydrogen gas plus oxygen and it's giving two waters. 373 00:26:20,000 --> 00:26:24,000 So it's basically burning hydrogen. And I think most of you know what 374 00:26:24,000 --> 00:26:29,000 would happen if I had a mixture of hydrogen and oxygen up here and 375 00:26:29,000 --> 00:26:34,000 chucked a match at it or something. We'd have a massive explosion. 376 00:26:34,000 --> 00:26:38,000 And, in fact, that's why there are 50 kilocalories of energy released 377 00:26:38,000 --> 00:26:42,000 when that happens. And so on an energy sort of diagram, 378 00:26:42,000 --> 00:26:47,000 these free energy diagrams where we had the two NADH here plus the 379 00:26:47,000 --> 00:26:51,000 molecule of oxygen, and down here we have the two 380 00:26:51,000 --> 00:26:55,000 molecules of water, there is this. If that happened in 381 00:26:55,000 --> 00:27:00,000 one step it would be a huge amount of energy. 382 00:27:00,000 --> 00:27:04,000 No cell or organism on earth has figured out how to do that in one 383 00:27:04,000 --> 00:27:08,000 step. And I think some of the textbooks liken it to sort of say it 384 00:27:08,000 --> 00:27:12,000 would be like setting a stick of dynamite off in the cell. 385 00:27:12,000 --> 00:27:16,000 And this is where one of these things that seemed probably like a 386 00:27:16,000 --> 00:27:20,000 kind of uninteresting thermodynamic property, to some extent, 387 00:27:20,000 --> 00:27:24,000 becomes to be really important in understanding biology. 388 00:27:24,000 --> 00:27:28,000 And that is the fact that remember I said this drop in energy from 389 00:27:28,000 --> 00:27:32,000 reactants to products was a thermodynamic property? 390 00:27:32,000 --> 00:27:35,000 It didn't matter whether like you skied straight down the hill or you 391 00:27:35,000 --> 00:27:38,000 came down in a series of little things, you still got the same 392 00:27:38,000 --> 00:27:42,000 amount of energy release going from here down to there. 393 00:27:42,000 --> 00:27:45,000 So that's, in essence, what the cell does, is it takes this 394 00:27:45,000 --> 00:27:48,000 energy and it breaks it into little packages that it's able to manage in 395 00:27:48,000 --> 00:27:52,000 a chemical way. And so instead of coming down, 396 00:27:52,000 --> 00:27:55,000 and once it comes down in a series of steps, and every time it flows 397 00:27:55,000 --> 00:27:59,000 partway down hill it does something. 398 00:27:59,000 --> 00:28:04,000 And what it does is it passes two electrons to some kind of carrier. 399 00:28:04,000 --> 00:28:09,000 And then the two electrons flow downhill a little bit to another one 400 00:28:09,000 --> 00:28:15,000 and then to another one. And what happens when these 401 00:28:15,000 --> 00:28:20,000 electrons are flowing downhill, though, is that they drive that 402 00:28:20,000 --> 00:28:26,000 proton pump. So a proton pump takes the proton from being on the outside 403 00:28:26,000 --> 00:28:31,000 to the inside. And when the two electrons drop 404 00:28:31,000 --> 00:28:37,000 down to the next level another H+ goes from out to H+ in, again here. 405 00:28:37,000 --> 00:28:49,000 And if you understood what I said 406 00:28:49,000 --> 00:28:54,000 before, what the cell can now do is it can make three ATPs by using that 407 00:28:54,000 --> 00:28:59,000 ATP synthesis, and now taking advantage of those 408 00:28:59,000 --> 00:29:05,000 three pump proteins and making them into three ATPs. 409 00:29:05,000 --> 00:29:11,000 And then at the end of the day what happens then is these two electrons 410 00:29:11,000 --> 00:29:18,000 get together with two hydrogen ions plus I'll show it as a half of an 411 00:29:18,000 --> 00:29:26,000 oxygen here to give you water. And what's happening up here is, 412 00:29:26,000 --> 00:29:34,000 in essence, the molecule of sugar, C6H1206 is being burned with six 413 00:29:34,000 --> 00:29:42,000 molecules of water plus this to give you six molecules of C02 plus six 414 00:29:42,000 --> 00:29:50,000 molecules of ATP so that the cell is essentially achieving the same kind 415 00:29:50,000 --> 00:29:58,000 of thing by this process as if it had burned it in oxygen. 416 00:29:58,000 --> 00:30:06,000 There's that much potential energy released. And the total amount of 417 00:30:06,000 --> 00:30:14,000 this change in the d-prime zero is something out of the order of I 418 00:30:14,000 --> 00:30:22,000 think it's minus 686 kilocalories per mole. It's able to capture 419 00:30:22,000 --> 00:30:30,000 respiration. It captures about 60% of that energy as ATP. 420 00:30:30,000 --> 00:30:38,000 This process of fermentation, this is what these processes that 421 00:30:38,000 --> 00:30:46,000 happen in the absence of oxygen are known as fermentation. 422 00:30:46,000 --> 00:30:54,000 As you can see, there much less efficient. It gets more like 3. 423 00:30:54,000 --> 00:31:01,000 % of the energy captured as ATP. So you can see when oxygen arose in 424 00:31:01,000 --> 00:31:05,000 the environment what a huge boom it was to life because you could get a 425 00:31:05,000 --> 00:31:09,000 lot more energy for the same amount of starting material. 426 00:31:09,000 --> 00:31:13,000 There is one thing, though, that the cell has to do. In order 427 00:31:13,000 --> 00:31:17,000 to do this it has to do some more chemistry because it has to take 428 00:31:17,000 --> 00:31:21,000 those pyruvates and it has to somehow run them through this thing 429 00:31:21,000 --> 00:31:25,000 that I'm called the citric acid cycle and oxidative 430 00:31:25,000 --> 00:31:30,000 phosphorylation. Well, the oxidative phosphorylation 431 00:31:30,000 --> 00:31:34,000 is basically this chain I've diagramed for you here in a 432 00:31:34,000 --> 00:31:38,000 schematic way. And that's about the level you'll 433 00:31:38,000 --> 00:31:42,000 have to understand it. I mean physically it's going to be 434 00:31:42,000 --> 00:31:46,000 a bunch of proteins stuck in the membrane, and as the electrons get 435 00:31:46,000 --> 00:31:50,000 passed along they pump a proton as this pathway occurs. 436 00:31:50,000 --> 00:31:54,000 But the other thing the cells have to do is they have to take those two 437 00:31:54,000 --> 00:31:59,000 pyruvates and they have to burn them all the way down to C02 and water. 438 00:31:59,000 --> 00:32:03,000 And what's happening, if you remember that chain of 439 00:32:03,000 --> 00:32:07,000 oxidations, the carbons are being successfully oxidized all the way up 440 00:32:07,000 --> 00:32:11,000 to carbon dioxide. You cannot be anymore oxidized than 441 00:32:11,000 --> 00:32:15,000 that if you're carbon. What that must mean is something 442 00:32:15,000 --> 00:32:19,000 else is getting reduced. And what gets reduced, where the 443 00:32:19,000 --> 00:32:23,000 electrons go is NADHs. So once oxygen became available in 444 00:32:23,000 --> 00:32:27,000 the atmosphere then the name of the game was to take those two pyruvates 445 00:32:27,000 --> 00:32:31,000 and to somehow burn them all the way down to here, and therefore generate 446 00:32:31,000 --> 00:32:36,000 as much NADH as possible. And if you can make some ATPs along 447 00:32:36,000 --> 00:32:40,000 the way well and good. So there had to be a whole other 448 00:32:40,000 --> 00:32:44,000 set of chemical reactions that's every bit as complicated as 449 00:32:44,000 --> 00:32:48,000 glycolysis that emerged in nature, that carried out that job. And this 450 00:32:48,000 --> 00:32:52,000 time I'm not going to take you through all of the chemical 451 00:32:52,000 --> 00:32:56,000 structures. You can look at it in your textbook and stuff, 452 00:32:56,000 --> 00:33:00,000 but what I really want you to kind of take is to keep your eye on the 453 00:33:00,000 --> 00:33:05,000 number of carbons. If you look at the structure of 454 00:33:05,000 --> 00:33:10,000 pyruvate you'll see that it's three carbons, and it was this. 455 00:33:10,000 --> 00:33:15,000 Here's the keto group and an acidic group. So this is almost carbon 456 00:33:15,000 --> 00:33:21,000 dioxide. It's just one oxidative step away. So what happens in this 457 00:33:21,000 --> 00:33:26,000 cycle is that first the carbon dioxide is removed and this 458 00:33:26,000 --> 00:33:34,000 makes an acetate. 459 00:33:34,000 --> 00:33:38,000 In essence it's actually joined to something right here. 460 00:33:38,000 --> 00:33:43,000 And then this feeds into this thing called the citric acid cycle. 461 00:33:43,000 --> 00:33:48,000 And what it is, as I said, is it's a set of chemical reactions that are 462 00:33:48,000 --> 00:33:52,000 designed to take this pyruvate and burn it all the way to carbon 463 00:33:52,000 --> 00:33:57,000 dioxide and water, and to generate as many NADHs and 464 00:33:57,000 --> 00:34:02,000 ATPs as it can along the way. And, again, I wish it were simpler. 465 00:34:02,000 --> 00:34:06,000 It would be really nice if it just did it straight from acetate, 466 00:34:06,000 --> 00:34:10,000 but instead you'll see, if you look at the TCA cycle, 467 00:34:10,000 --> 00:34:14,000 the C4 compound. And this gets joined together to give us C6 468 00:34:14,000 --> 00:34:18,000 compound. And then it oxidizes one of these carbons to give a carbon 469 00:34:18,000 --> 00:34:22,000 dioxide. Now you're down to the C5. It does it again, another molecule 470 00:34:22,000 --> 00:34:26,000 of C02, you're at C4. And then it takes this C4 carbon 471 00:34:26,000 --> 00:34:30,000 skeleton, puts it through some transformations that enables this 472 00:34:30,000 --> 00:34:36,000 cycle to go all over again. So basically the C4 thing just goes 473 00:34:36,000 --> 00:34:42,000 through the cycle. It's a carrier that takes this C2 474 00:34:42,000 --> 00:34:48,000 right here, lets it be processed to give two molecules of C02. 475 00:34:48,000 --> 00:34:54,000 In that process it generates more NADH, a bit of ATP, 476 00:34:54,000 --> 00:35:00,000 and a kind of reduced carrier that you can think of for the moment 477 00:35:00,000 --> 00:35:07,000 largely equivalently to NADH. OK. Now, the entity in our cells 478 00:35:07,000 --> 00:35:15,000 that carries out this process of respiration -- 479 00:35:15,000 --> 00:35:24,000 Of respiration. 480 00:35:24,000 --> 00:35:28,000 So the citric acid cycle and the mitochondria is not actually in our 481 00:35:28,000 --> 00:35:32,000 cytoplasm, which is perhaps surprising, but all the enzymes for 482 00:35:32,000 --> 00:35:37,000 glycolysis are. So remember my simple diagram of the 483 00:35:37,000 --> 00:35:41,000 eukaryotic cell one the first day? We had the nucleus which is a 484 00:35:41,000 --> 00:35:45,000 membrane compartment that has the DNA inside it, 485 00:35:45,000 --> 00:35:49,000 and we'll have more to say about that in the next lecture of so, 486 00:35:49,000 --> 00:35:54,000 but then there were some organelles. And one of them I said was the 487 00:35:54,000 --> 00:35:58,000 mitochondrion. And I also said in the first 488 00:35:58,000 --> 00:36:02,000 lecture that there's pretty good evidence now that the mitochondrion 489 00:36:02,000 --> 00:36:06,000 arose by some earlier progenitor or ancestor of a eukaryotic cell 490 00:36:06,000 --> 00:36:11,000 capturing some kind of bacterium. It's actually one that's sort of 491 00:36:11,000 --> 00:36:15,000 kind of related to E. coli. It had double membranes. 492 00:36:15,000 --> 00:36:20,000 And the mitochondria actually still have some of their own DNA, 493 00:36:20,000 --> 00:36:24,000 but this is where all the energy, all the enzymes for the citric acid 494 00:36:24,000 --> 00:36:32,000 cycle -- 495 00:36:32,000 --> 00:36:37,000 -- and oxidative phosphorylation are found in the inside of the 496 00:36:37,000 --> 00:36:42,000 mitochondrion, instead of something that probably 497 00:36:42,000 --> 00:36:47,000 used to be a free-living bacterium. In contrast, the enzymes for 498 00:36:47,000 --> 00:36:52,000 glycolysis are in what's called the cytoplasm of the cell, 499 00:36:52,000 --> 00:36:57,000 which is sort of the main part of the insides of the cell. 500 00:36:57,000 --> 00:37:00,000 So you see even here this sort of evolutionary history that I kind of 501 00:37:00,000 --> 00:37:04,000 put out across the board. The enzymes for glycolysis arose so 502 00:37:04,000 --> 00:37:07,000 early in evolution they're in the cytoplasm of virtually every 503 00:37:07,000 --> 00:37:11,000 organism on earth. Eukaryotic cells manage to figure 504 00:37:11,000 --> 00:37:14,000 out how to get 18 times more energy for a molecule of sugar, 505 00:37:14,000 --> 00:37:18,000 but in order to do it they kind of had to cheat. They took a bacterium 506 00:37:18,000 --> 00:37:21,000 and figured out how to do it, put it inside our own cells, and 507 00:37:21,000 --> 00:37:25,000 then it's able to run now, it's become a part of our cell and 508 00:37:25,000 --> 00:37:29,000 it's where all the process takes place. 509 00:37:29,000 --> 00:37:34,000 And if you look at a structure of a mitochondrion it's actually got two 510 00:37:34,000 --> 00:37:40,000 membranes. You just saw a picture of that, an E. 511 00:37:40,000 --> 00:37:45,000 coli. And it's sort of involuted a little bit like this. 512 00:37:45,000 --> 00:37:51,000 And so this is an outer membrane of a mitochondrion. 513 00:37:51,000 --> 00:37:57,000 So I'm basically taking this and blowing it up. So this 514 00:37:57,000 --> 00:38:02,000 is a mitochondrion. And the in and the out, 515 00:38:02,000 --> 00:38:07,000 in this case there's an inner membrane. This is where the proton 516 00:38:07,000 --> 00:38:11,000 pump is located. This is where the ATP synthesis is 517 00:38:11,000 --> 00:38:16,000 located. And when a mitochondrion is working what it's doing is it's 518 00:38:16,000 --> 00:38:21,000 pumping hydrogen ions from its inside, which is more or less the 519 00:38:21,000 --> 00:38:25,000 equivalent of its cytoplasm, it's whatever used to be the 520 00:38:25,000 --> 00:38:30,000 cytoplasm of the original bacterium, outside into the space between the 521 00:38:30,000 --> 00:38:35,000 inner and outer membranes. And then it generates ATP by flowing 522 00:38:35,000 --> 00:38:39,000 back down. OK. So there are a couple of sort of 523 00:38:39,000 --> 00:38:43,000 things that affect your life that come out of this. 524 00:38:43,000 --> 00:38:48,000 One of them is understanding the ìfreshman fiveî or ìfreshman 525 00:38:48,000 --> 00:38:52,000 fifteenî or whatever it is. You come to college and all of a 526 00:38:52,000 --> 00:38:57,000 sudden you put on a lot of weight. Sometimes you can figure it out 527 00:38:57,000 --> 00:39:01,000 because you ate a lot of Ben & Jerry's or a lot of chocolate bars 528 00:39:01,000 --> 00:39:06,000 or something and didn't do as much exercise. 529 00:39:06,000 --> 00:39:09,000 Another one of you said, gee, now I understand why I'm not 530 00:39:09,000 --> 00:39:12,000 putting on any weight. I do water polo, and I guess I must 531 00:39:12,000 --> 00:39:15,000 be burning up everything I eat as energy. And you're right, 532 00:39:15,000 --> 00:39:18,000 but now I can show you at a molecular level what's happening. 533 00:39:18,000 --> 00:39:21,000 So what happens, your body, everything is regulated, 534 00:39:21,000 --> 00:39:25,000 and it can tell if it needs, cells in your body can tell if it 535 00:39:25,000 --> 00:39:28,000 needs energy. If it needs energy and you eat sugar it runs right 536 00:39:28,000 --> 00:39:32,000 through here and it makes ATP. But if you've eaten enough and the 537 00:39:32,000 --> 00:39:36,000 things that monitor your body say that you've got enough ATP then it 538 00:39:36,000 --> 00:39:41,000 doesn't keep this process going. Instead what it does is it stops it 539 00:39:41,000 --> 00:39:46,000 and says instead we're going to put something away for a rainy day, 540 00:39:46,000 --> 00:39:50,000 which you could see makes a lot of sense in evolution. 541 00:39:50,000 --> 00:39:55,000 You know, if things are good you just kill the mammoth and you can 542 00:39:55,000 --> 00:40:00,000 all eat well for a while or something. 543 00:40:00,000 --> 00:40:05,000 It would make sense for your body to be able to pack stuff away. 544 00:40:05,000 --> 00:40:10,000 So what it does is it intercepts the process at this level, 545 00:40:10,000 --> 00:40:16,000 at this C2 level. The pyruvate gets to here, and instead of being run 546 00:40:16,000 --> 00:40:21,000 through this in ATP this acetate or acetyl moiety which is a C2 gets run 547 00:40:21,000 --> 00:40:26,000 through a cycle of successive two carbon additions. 548 00:40:26,000 --> 00:40:32,000 And what comes out of that are fatty acids. It also takes 549 00:40:32,000 --> 00:40:43,000 three phosphor -- 550 00:40:43,000 --> 00:40:47,000 -- glyceraldehyde. You might remember that. 551 00:40:47,000 --> 00:40:52,000 That's one of the products stuck in the middle of that glycolytic 552 00:40:52,000 --> 00:40:57,000 pathway. So this is a three carbon compound. It makes it into glycerol 553 00:40:57,000 --> 00:41:02,000 which is a three carbon compound. And if you look back when we talked 554 00:41:02,000 --> 00:41:07,000 about lipids a couple of lectures ago you'll realize what happens if 555 00:41:07,000 --> 00:41:12,000 you combine fatty acids with glycerol then you've got fats. 556 00:41:12,000 --> 00:41:17,000 And, in essence, I mean that's what happens. We put on weight if we eat 557 00:41:17,000 --> 00:41:22,000 more than we're burning. Our bodies say, OK, I've got as 558 00:41:22,000 --> 00:41:27,000 much ATP as we need. I'm going to take some of that ATP 559 00:41:27,000 --> 00:41:31,000 and use it in this kind of way. Just a second here. 560 00:41:31,000 --> 00:41:37,000 I'm going to see -- 561 00:41:37,000 --> 00:41:41,000 So there's another thing that happens, a physiological thing that 562 00:41:41,000 --> 00:41:45,000 we experience. Well, we seem to be stuck at the 563 00:41:45,000 --> 00:41:49,000 moment. I won't worry about it. I've run some marathons. I don't 564 00:41:49,000 --> 00:41:53,000 know. A few of you may have. If not almost all of you have heard 565 00:41:53,000 --> 00:41:57,000 about ìhitting the wallî. And it happens generally around 21, 566 00:41:57,000 --> 00:42:01,000 22, 23 miles. It depends on the condition you're 567 00:42:01,000 --> 00:42:05,000 doing. And physiologically I have experienced, this is amazing, 568 00:42:05,000 --> 00:42:08,000 I understand why they call it hitting the wall. 569 00:42:08,000 --> 00:42:11,000 You're running along and you think, boy, I'm tired but I'm doing OK. 570 00:42:11,000 --> 00:42:15,000 And in a space of a quarter of mile it's like you banged into a brick 571 00:42:15,000 --> 00:42:18,000 wall. And you can sort of keep yourself going but it's a profound 572 00:42:18,000 --> 00:42:22,000 physiological change. And what has happened at that point 573 00:42:22,000 --> 00:42:25,000 is you've run out of sugar to burn. Now, I've told you, you can take 574 00:42:25,000 --> 00:42:29,000 sugars and you can polymerize them into glycogen. 575 00:42:29,000 --> 00:42:32,000 That's one of those polymers with alpha 1,4 linkages. 576 00:42:32,000 --> 00:42:36,000 That's a sugar storage molecule. So if you start running, you 577 00:42:36,000 --> 00:42:39,000 carbo-load, you try and get as much sugar into glycogen as you possibly 578 00:42:39,000 --> 00:42:43,000 can. You start running the marathon. Your body starts taking that 579 00:42:43,000 --> 00:42:46,000 glycogen and breaking it down into sugars. It runs it through that 580 00:42:46,000 --> 00:42:50,000 process. What happens when you hit the wall as a human body is most 581 00:42:50,000 --> 00:42:53,000 human bodies are designed so that you run out of sugar. 582 00:42:53,000 --> 00:42:57,000 You just cannot carbo-load enough to do 26 miles. 583 00:42:57,000 --> 00:43:01,000 You can get through 20 or something there. 584 00:43:01,000 --> 00:43:04,000 And when you run out of that what happens then is your body can no 585 00:43:04,000 --> 00:43:08,000 longer burn sugar so it switched to burning fatty acids. 586 00:43:08,000 --> 00:43:11,000 And that's less efficient and you really feel it. 587 00:43:11,000 --> 00:43:15,000 If any of you have done long endurance things or triathlons or 588 00:43:15,000 --> 00:43:19,000 anything you've experienced that change. The final thing, 589 00:43:19,000 --> 00:43:22,000 just to close for the lecture for today, is one thing you can 590 00:43:22,000 --> 00:43:26,000 appreciate. Yeast is somebody that can do both of these things, 591 00:43:26,000 --> 00:43:30,000 right? It can either grow anaeorbically. 592 00:43:30,000 --> 00:43:37,000 And it makes C02 plus ethanol. Or it can grow aerobically. And 593 00:43:37,000 --> 00:43:44,000 this gives it two ATPs per glucose and this gives it 36 ATPs per 594 00:43:44,000 --> 00:43:51,000 glucose. So if you were a yeast, you would have to somehow regulate 595 00:43:51,000 --> 00:43:59,000 the rate of glycolysis depending on whether there was oxygen 596 00:43:59,000 --> 00:44:06,000 around or not. And it's very tricky and neat way it 597 00:44:06,000 --> 00:44:13,000 happens. There's an enzyme, that wouldn't surprise you, that has 598 00:44:13,000 --> 00:44:19,000 an active site for one of the key steps in the pathway where it's 599 00:44:19,000 --> 00:44:26,000 going from fructose-1-phosphate, fructose-6-phosphate to fructose-1, 600 00:44:26,000 --> 00:44:32,000 -diphosphate. That's one of the intermediate steps 601 00:44:32,000 --> 00:44:36,000 in glycolysis. It's the enzyme that turns out to 602 00:44:36,000 --> 00:44:41,000 be rate-limiting. You can control passage of 603 00:44:41,000 --> 00:44:45,000 something through a pathway by just making one step rate-limiting. 604 00:44:45,000 --> 00:44:50,000 So this has a place for the fructose 6 phosphate plus ATP to 605 00:44:50,000 --> 00:44:54,000 bind, and it catalyzes the formation of the fructose-1, 606 00:44:54,000 --> 00:44:59,000 -diphosphate. But that enzyme has to work at different rates depending 607 00:44:59,000 --> 00:45:04,000 on whether the cell is aerobic or anaerobic. 608 00:45:04,000 --> 00:45:11,000 So what it does is there's a second binding site over here. 609 00:45:11,000 --> 00:45:18,000 And if it binds ATP, this speeds up, excuse me, slows down the rate 610 00:45:18,000 --> 00:45:25,000 that's catalyzed over here. And that's what you'd expect. 611 00:45:25,000 --> 00:45:32,000 If it's got enough ATP it doesn't need to run glycolysis as fast. 612 00:45:32,000 --> 00:45:40,000 And it also binds AMP or ADP, and that speeds up the rate. 613 00:45:40,000 --> 00:45:43,000 So the yeast are able to monitor, to sort of have a control on how 614 00:45:43,000 --> 00:45:47,000 fast sugar flows through this glycolytic pathway. 615 00:45:47,000 --> 00:45:51,000 And what they're really doing is they're monitoring do I have enough 616 00:45:51,000 --> 00:45:55,000 ATP or not? And so if they're running with respiration, 617 00:45:55,000 --> 00:45:59,000 making lots of stuff, they don't need to do glycolysis as fast. 618 00:45:59,000 --> 00:46:03,000 If they're anaerobic they have to run it 18 times as fast to get the 619 00:46:03,000 --> 00:46:08,000 same amount of energy. OK? We'll begin the next lecture 620 00:46:08,000 --> 00:46:12,000 with photosynthesis and then we're going to get into a bunch of 621 00:46:12,000 --> 00:46:15,000 molecular biology. OK?