WEBVTT 00:00:11.293 --> 00:00:12.230 PROFESSOR: OK. 00:00:12.230 --> 00:00:16.370 So I need to move on a little bit now, and I want to talk 00:00:16.370 --> 00:00:24.540 about in fact, the earlier way that nature developed to make 00:00:24.540 --> 00:00:27.930 energy using proton gradients. 00:00:27.930 --> 00:00:30.940 And this part actually preceded the development of 00:00:30.940 --> 00:00:32.700 respiration as you'll see. 00:00:32.700 --> 00:00:37.930 It's what I somewhat flippantly refer to as 00:00:37.930 --> 00:00:41.970 photosynthesis release 1. 00:00:41.970 --> 00:00:44.060 In my first lecture, when I was giving 00:00:44.060 --> 00:00:49.180 you a sketch of evolution. 00:00:49.180 --> 00:00:52.480 Who knows, I mean these are very rough numbers, but may 00:00:52.480 --> 00:00:57.330 have evolved about $3.4 billion years ago and early 00:00:57.330 --> 00:01:00.715 life had begun to exhaust this sea of chemicals 00:01:00.715 --> 00:01:02.970 that had been produced. 00:01:02.970 --> 00:01:08.630 And it's known as cyclic photophosphorylation. 00:01:17.400 --> 00:01:20.690 And it's a way of taking the energy in 00:01:20.690 --> 00:01:26.470 sunlight and making ATP. 00:01:26.470 --> 00:01:30.540 So whatever organism figured this out, this was 00:01:30.540 --> 00:01:31.590 a really big deal. 00:01:31.590 --> 00:01:35.100 Because now instead of having to use the sort of natural 00:01:35.100 --> 00:01:39.350 reserves like the way the food around is a depleting resource 00:01:39.350 --> 00:01:41.950 just like our petroleum reserves, this was able to 00:01:41.950 --> 00:01:44.780 take the abundantly-available sunlight and 00:01:44.780 --> 00:01:47.280 use it to make energy. 00:01:47.280 --> 00:01:49.840 So that's the principle. 00:01:49.840 --> 00:01:53.590 It uses the energy in sunlight, and the way it does 00:01:53.590 --> 00:02:02.580 it, is it uses the sunlight to establish a proton gradient 00:02:02.580 --> 00:02:06.290 just as we've been discussing earlier, and then uses 00:02:06.290 --> 00:02:11.160 that to make ATP. 00:02:11.160 --> 00:02:18.630 And there's a special molecule that's involved in absorbing 00:02:18.630 --> 00:02:21.620 the energy from the sunlight. 00:02:21.620 --> 00:02:24.560 We've all heard it I'm sure, chlorophyll. 00:02:24.560 --> 00:02:26.870 There are a couple of two major 00:02:26.870 --> 00:02:29.400 variants of this molecule. 00:02:29.400 --> 00:02:32.810 Here's one of them, chlorophyll a, and you don't 00:02:32.810 --> 00:02:34.150 have to memorize the structure. 00:02:34.150 --> 00:02:35.850 But I want you to notice a couple of things. 00:02:35.850 --> 00:02:40.800 One is there's a metal in the middle, a magnesium, and then 00:02:40.800 --> 00:02:49.800 it's coordinated with the cyclic ring system. 00:02:49.800 --> 00:02:52.780 And notice all the conjugated double bonds. 00:02:52.780 --> 00:02:58.090 So this chlorophyll was tuned to absorb energy from the 00:02:58.090 --> 00:02:59.845 visible range of sunlight. 00:02:59.845 --> 00:03:03.510 And when it absorbs a photon of energy, it kicks an 00:03:03.510 --> 00:03:06.450 electron up to a higher orbital. 00:03:06.450 --> 00:03:09.940 And if the electron's in a higher orbital, it's more 00:03:09.940 --> 00:03:12.130 easily lost. 00:03:12.130 --> 00:03:13.810 And this has a consequence. 00:03:13.810 --> 00:03:20.020 So the way this system works is you have a molecule of 00:03:20.020 --> 00:03:23.580 chlorophyll, that's what I'm abbreviating here. 00:03:23.580 --> 00:03:26.420 Oh, let me tell you something else too. 00:03:26.420 --> 00:03:29.260 It's more sophisticated than this. 00:03:29.260 --> 00:03:32.850 So this is the molecule that absorbs a particular 00:03:32.850 --> 00:03:34.650 wavelength from sunlight. 00:03:34.650 --> 00:03:40.910 But this is embedded in a multi-protein structure that 00:03:40.910 --> 00:03:46.330 has a bunch of other molecules that absorb at different 00:03:46.330 --> 00:03:49.770 wavelengths and then funnel that energy down to the one 00:03:49.770 --> 00:03:51.810 the chlorophyll comes in. 00:03:51.810 --> 00:03:55.560 So in fact, the whole thing is like a big antenna that's able 00:03:55.560 --> 00:03:58.420 to absorb quite a bit of energy from different 00:03:58.420 --> 00:04:01.310 wavelengths in sunlight and get it down to the 00:04:01.310 --> 00:04:02.470 chlorophyll. 00:04:02.470 --> 00:04:10.510 When the chlorophyll absorbs energy, it goes up to an 00:04:10.510 --> 00:04:12.670 excited state. 00:04:12.670 --> 00:04:14.910 And as I said, now that the electron's in a higher 00:04:14.910 --> 00:04:17.640 orbital, it's lost more easily. 00:04:17.640 --> 00:04:20.649 So this has become a better reducing agent. 00:04:20.649 --> 00:04:23.670 It's able to give its electrons to things that it 00:04:23.670 --> 00:04:26.450 couldn't do down in this energy state. 00:04:26.450 --> 00:04:29.130 So we come down one of these thermodynamic hills that 00:04:29.130 --> 00:04:31.850 you're hopefully starting to get used to where it comes 00:04:31.850 --> 00:04:36.180 down in little hops to a carrier that has this set 00:04:36.180 --> 00:04:39.580 level of energy, down, free energy, down. 00:04:43.720 --> 00:04:47.650 And similarly, to the principle that we talked about 00:04:47.650 --> 00:04:54.210 in respiration, a proton is pumped from what I'm going to 00:04:54.210 --> 00:05:00.380 say, in this case, I'll show you what I mean, but I'll say 00:05:00.380 --> 00:05:06.140 from a proton that's out to a proton that's in. 00:05:06.140 --> 00:05:11.310 And by doing that, it establishes a proton gradient, 00:05:11.310 --> 00:05:13.230 and that gives rise to ATP. 00:05:17.080 --> 00:05:20.740 At the end of this cycle, we have this chlorophyll minus 00:05:20.740 --> 00:05:22.170 the electrons. 00:05:22.170 --> 00:05:24.070 These come, flow back. 00:05:24.070 --> 00:05:28.610 That's why it's called cyclic photophosphorylation. 00:05:28.610 --> 00:05:33.330 The electrons go through these carriers, and then they return 00:05:33.330 --> 00:05:34.760 to chlorophyll. 00:05:34.760 --> 00:05:36.645 So wonderful system. 00:05:43.500 --> 00:05:45.670 I seemed to have accidentally advanced this. 00:05:45.670 --> 00:05:46.260 OK. 00:05:46.260 --> 00:05:51.550 There are still bacteria around that run this system. 00:05:51.550 --> 00:05:56.170 So if you remember, we talked about biosynthesis, the need 00:05:56.170 --> 00:05:56.625 for energy. 00:05:56.625 --> 00:05:57.390 Well, here we are. 00:05:57.390 --> 00:05:58.360 We got ATP. 00:05:58.360 --> 00:06:00.430 But there is something else hopefully you now appreciate 00:06:00.430 --> 00:06:06.400 in that is that ATP is not enough to take carbon dioxide 00:06:06.400 --> 00:06:09.030 and make it into sugars or carbon compounds. 00:06:09.030 --> 00:06:11.190 We need a source of reducing power as well. 00:06:11.190 --> 00:06:14.460 Because remember, carbon dioxide is the most oxidized 00:06:14.460 --> 00:06:16.140 form of carbon. 00:06:16.140 --> 00:06:21.080 So these early organism solved it by making reducing power 00:06:21.080 --> 00:06:24.450 from another source. 00:06:24.450 --> 00:06:29.260 Many of them used hydrogen sulfide as a source, and they 00:06:29.260 --> 00:06:32.150 used NADP plus. 00:06:32.150 --> 00:06:36.090 Now, this is a minor variation of NADH. 00:06:36.090 --> 00:06:38.370 It's got one more phosphate on it. 00:06:38.370 --> 00:06:40.730 You can look it up in your book. 00:06:40.730 --> 00:06:45.440 This variant of NAD is used preferentially for 00:06:45.440 --> 00:06:46.880 biosynthetic purposes. 00:06:46.880 --> 00:06:50.660 But everything I've told you about NAD in terms of banking 00:06:50.660 --> 00:06:52.660 electrons applies here. 00:06:52.660 --> 00:06:58.500 So the electrons from here are grabbed. 00:06:58.500 --> 00:07:06.640 The cell makes NADPH, which you can use as reducing power 00:07:06.640 --> 00:07:07.890 for biosynthesis. 00:07:10.660 --> 00:07:14.320 You get elemental sulfur and hydrogen ions. 00:07:14.320 --> 00:07:21.160 So this process an organism that used this kind of 00:07:21.160 --> 00:07:24.680 photophosphorylation to make ATP would get its reducing 00:07:24.680 --> 00:07:27.930 power through a process something like this. 00:07:27.930 --> 00:07:32.500 And then it could make sugars, and then from that point on, 00:07:32.500 --> 00:07:35.170 they can be used to make all the other 00:07:35.170 --> 00:07:36.860 molecules that you need. 00:07:36.860 --> 00:07:40.170 The key thing is to get from the carbon dioxide down into a 00:07:40.170 --> 00:07:43.280 more reduced form of carbon. 00:07:43.280 --> 00:07:48.050 So that works pretty well. 00:07:48.050 --> 00:07:53.580 However, a better system came up involved in evolution. 00:07:53.580 --> 00:08:00.180 This was the one I again somewhat flippantly called 00:08:00.180 --> 00:08:06.690 photosynthesis release 2, when I was talking. 00:08:06.690 --> 00:08:14.380 This is known as, probably came up who knows again but 00:08:14.380 --> 00:08:27.900 maybe 3 billion years ago, and it's known as noncyclic 00:08:27.900 --> 00:08:29.530 photophosphorylation. 00:08:29.530 --> 00:08:32.750 And what's important about this system and why it's an 00:08:32.750 --> 00:08:37.270 improvement over the other, is it uses the energy in sunlight 00:08:37.270 --> 00:08:41.820 to make ATP just as we've learned, but it also uses the 00:08:41.820 --> 00:08:47.180 energy in sunlight to make NADPH. 00:08:47.180 --> 00:08:52.010 So in other words, this second version gives the cell simply 00:08:52.010 --> 00:08:55.330 from the energy in sunlight everything it needs to take 00:08:55.330 --> 00:09:00.160 carbon dioxide and make it into organic compounds. 00:09:00.160 --> 00:09:03.190 And it's a pretty cool system evolutionarily. 00:09:03.190 --> 00:09:07.490 It's built on the older one, the first arising one. 00:09:07.490 --> 00:09:11.350 You'll still see the elements of the present but with a new 00:09:11.350 --> 00:09:16.090 variation added in, very much the way we do design when 00:09:16.090 --> 00:09:16.795 you're doing engineering. 00:09:16.795 --> 00:09:19.770 You get something that's working, and you can use that 00:09:19.770 --> 00:09:22.820 as a basis to move to a new and improved version. 00:09:22.820 --> 00:09:24.925 And naturally, if you get a new and improved version, and 00:09:24.925 --> 00:09:27.270 you get a little advantage over your neighbors, natural 00:09:27.270 --> 00:09:32.760 selection makes sure that that better system gets 00:09:32.760 --> 00:09:34.160 established. 00:09:34.160 --> 00:09:37.110 So here's here how this noncyclic 00:09:37.110 --> 00:09:38.890 photophosphorylation starts. 00:09:38.890 --> 00:09:45.535 We take a chlorophyll and it absorbs the quantum of energy, 00:09:45.535 --> 00:09:50.650 and it kicks itself up to an excited state. 00:09:50.650 --> 00:09:56.230 The chlorophyll, as before, electrons come down, 00:09:56.230 --> 00:09:58.760 energetically downhill and remember that theme. 00:09:58.760 --> 00:10:03.150 I keep saying that's at least thermodynamic properties. 00:10:03.150 --> 00:10:04.910 If we think about free energy, it doesn't matter 00:10:04.910 --> 00:10:05.920 what path you take. 00:10:05.920 --> 00:10:09.280 Whether you come shooting right down or you come down 00:10:09.280 --> 00:10:12.120 through it, you get the same energy back. 00:10:12.120 --> 00:10:14.120 What's amazing about the system, if it didn't have all 00:10:14.120 --> 00:10:16.790 this extra apparatus, you'd kick up the electron, and then 00:10:16.790 --> 00:10:18.880 it would just come right back and you'd get a little 00:10:18.880 --> 00:10:20.130 radiated, a little energy given off. 00:10:20.130 --> 00:10:21.480 We wouldn't have accomplished anything. 00:10:21.480 --> 00:10:25.680 And what's terrific about this photophosphorylation system, 00:10:25.680 --> 00:10:28.780 it's able to capture the energy that's in that excited 00:10:28.780 --> 00:10:29.780 chlorophyll. 00:10:29.780 --> 00:10:35.170 So at this point, and as it's coming down as I said, we have 00:10:35.170 --> 00:10:40.090 H plus going from H plus, H plus in. 00:10:40.090 --> 00:10:43.090 I'll give you a picture of what I mean by that. 00:10:43.090 --> 00:10:46.380 Then we're getting ATP made. 00:10:46.380 --> 00:10:50.920 So this time the difference is instead of the electrons going 00:10:50.920 --> 00:10:54.940 back to that chlorophyll, it was missing its electrons, the 00:10:54.940 --> 00:11:00.710 electrons, instead, go to a chlorophyll, which is at a 00:11:00.710 --> 00:11:04.000 somewhat higher energy level than the first one. 00:11:04.000 --> 00:11:09.295 And it has just absorbed quantum of energy, and it's 00:11:09.295 --> 00:11:16.720 kicked itself up to an even more excited state. 00:11:16.720 --> 00:11:22.390 And these electrons from this system come on and fill up 00:11:22.390 --> 00:11:24.770 this chlorophyll. 00:11:24.770 --> 00:11:33.495 So this one over here is called photosystem II. 00:11:33.495 --> 00:11:36.530 And the term used in the field to describe what I'm about to 00:11:36.530 --> 00:11:47.650 tell you here is now called photosystem I. 00:11:47.650 --> 00:11:54.550 So what we have now from this system is an excited molecule, 00:11:54.550 --> 00:11:59.190 chlorophyll, that's even more excited than we were before. 00:11:59.190 --> 00:12:03.000 And so it's even more able to give off its electrons. 00:12:03.000 --> 00:12:04.750 It has more reducing power. 00:12:04.750 --> 00:12:07.240 In fact, it has enough reducing power that it can 00:12:07.240 --> 00:12:09.440 reduce NADP. 00:12:09.440 --> 00:12:22.030 So NADP plus, electrons coming downhill, you get NADPH. 00:12:25.690 --> 00:12:32.000 So here we are, reducing power made by using the energy in 00:12:32.000 --> 00:12:38.660 sunlight, ATP made using the energy in sunlight. 00:12:38.660 --> 00:12:43.300 So by just using this noncyclic photophosphorylation 00:12:43.300 --> 00:12:48.930 system, the cell's got what it needs to take carbon dioxide 00:12:48.930 --> 00:12:51.740 and put it through a sequence of reactions that will let it 00:12:51.740 --> 00:12:53.600 make sugars and other things. 00:12:53.600 --> 00:12:56.020 In this course, I don't have enough time to go through the 00:12:56.020 --> 00:12:58.470 biosynthetic pathway. 00:12:58.470 --> 00:12:59.940 It's in your textbooks. 00:12:59.940 --> 00:13:02.455 You might find it interesting to look at. 00:13:02.455 --> 00:13:04.490 We're not making a big issue of it in there. 00:13:04.490 --> 00:13:07.500 But it exists, and you can see that it obviously exists. 00:13:07.500 --> 00:13:11.550 So there's one more wrinkle here which might be sort of 00:13:11.550 --> 00:13:15.615 eerily reminiscent of one of the issue I posed for you when 00:13:15.615 --> 00:13:19.670 I asked about whether we could just keep on doing glycolysis. 00:13:19.670 --> 00:13:21.240 I can't just let the system run. 00:13:24.840 --> 00:13:27.800 I forgot about something so far. 00:13:27.800 --> 00:13:31.630 Over here, this guy lost an electron. 00:13:31.630 --> 00:13:34.990 It can't get it back because the electrons went over there. 00:13:34.990 --> 00:13:36.720 They have to come from somewhere. 00:13:36.720 --> 00:13:40.170 Well, the energetics of the system now are such that it 00:13:40.170 --> 00:13:45.515 can get electrons from water. 00:13:45.515 --> 00:13:48.840 And what's left over when you take the electrons from water? 00:13:48.840 --> 00:13:54.040 We have half of an oxygen molecule. 00:13:54.040 --> 00:13:59.120 So here's the class. 00:13:59.120 --> 00:14:05.430 It's a waste product, if you will, from this very efficient 00:14:05.430 --> 00:14:08.120 noncyclic photophosphorylation system, but 00:14:08.120 --> 00:14:09.880 it's molecular oxygen. 00:14:09.880 --> 00:14:13.020 And it was when this system developed that we started to 00:14:13.020 --> 00:14:17.810 have oxygen appear in this world. 00:14:17.810 --> 00:14:25.260 The organelle that carries out photosynthesis, actually, the 00:14:25.260 --> 00:14:30.940 first organisms that learned how to do this is called 00:14:30.940 --> 00:14:35.060 cyanobacteria, which they sometimes sort of rather 00:14:35.060 --> 00:14:36.600 incorrectly call blue-green algae 00:14:36.600 --> 00:14:39.650 because they're bacteria. 00:14:39.650 --> 00:14:42.710 But you see cyanobacteria all the time. 00:14:42.710 --> 00:14:45.620 And similarly to what happened with the mitochondria, there's 00:14:45.620 --> 00:14:50.150 no abundant evidence that the way photosynthesis happens in 00:14:50.150 --> 00:14:56.700 plants is a cyanobacterium got trapped somehow inside a early 00:14:56.700 --> 00:14:59.280 plant cell, and is now a permanent part 00:14:59.280 --> 00:15:01.550 of the plant cell. 00:15:01.550 --> 00:15:02.800 And it's called a chloroplast. 00:15:06.560 --> 00:15:09.160 So it's derived from a bacterium. 00:15:09.160 --> 00:15:13.400 If you see plants are green. 00:15:13.400 --> 00:15:20.430 If you can look in and see the chloroplast inside, this shows 00:15:20.430 --> 00:15:21.920 the chloroplast coming in. 00:15:21.920 --> 00:15:23.870 And here's their basic structure. 00:15:23.870 --> 00:15:27.360 They too have a double membrane. 00:15:27.360 --> 00:15:29.100 They have an outer membrane. 00:15:29.100 --> 00:15:30.360 They have an inner membrane. 00:15:33.840 --> 00:15:38.910 They have a part that's called the stroma, and that's 00:15:38.910 --> 00:15:44.380 essentially, like the cytoplasm of a normal cell. 00:15:44.380 --> 00:15:49.130 And they have something in here called a lumen. 00:15:49.130 --> 00:15:53.360 It's a space, and the membrane that bounds it is a special 00:15:53.360 --> 00:15:59.040 membrane called a thylakoid membrane. 00:16:06.580 --> 00:16:11.070 And that gradient is established by pumping an 00:16:11.070 --> 00:16:17.060 electron from the stroma, which I called out, into the 00:16:17.060 --> 00:16:20.190 lumen, which I called in. 00:16:20.190 --> 00:16:24.540 Again, the point is this cell managed to establish a proton 00:16:24.540 --> 00:16:29.090 gradient, and it's able to make the chloroplast, able to 00:16:29.090 --> 00:16:33.230 establish a protein gradient, and make ATP. 00:16:33.230 --> 00:16:38.510 And there's a transmission micrograph of a chloroplast. 00:16:38.510 --> 00:16:42.900 You can see the thylakoid membranes inside. 00:16:42.900 --> 00:16:45.830 It's not too hard to imagine that that was, in fact, a 00:16:45.830 --> 00:16:47.660 cyanobacterium that got in there. 00:16:47.660 --> 00:16:49.980 And there's quite a bit of additional evidence that 00:16:49.980 --> 00:16:51.230 supports that.