0:00:00.350,0:00:03.720 - Let's delve into the world of mitochondria 0:00:03.720,0:00:07.250 which are probably my favorite organelle. 0:00:07.250,0:00:10.090 So let's just have a little review of what mitochondria are 0:00:10.090,0:00:11.450 and then we can delve a little bit deeper 0:00:11.450,0:00:13.390 into their structure. 0:00:13.390,0:00:15.990 So let's just think about a cell 0:00:15.990,0:00:18.490 and not just any cell, but a eukaryotic cell. 0:00:18.490,0:00:20.120 So that's the cellular membrane 0:00:20.120,0:00:23.650 and when people say a eukaryote or a eukaryotic cell 0:00:23.650,0:00:27.290 they most typically say, "Oh! That must have its nuclear DNA 0:00:27.290,0:00:31.020 "in a membrane-bound nucleus." and that would be true, 0:00:31.020,0:00:33.690 so let's draw our membrane-bound nucleus. 0:00:33.690,0:00:35.220 That's our nuclear membrane. 0:00:35.220,0:00:38.090 You have your DNA in here, 0:00:38.090,0:00:39.990 so let's draw some DNA. 0:00:39.990,0:00:42.190 But when we talk about eukaryotic cells, 0:00:42.190,0:00:44.990 we're not just talking about a membrane-bound nucleus, 0:00:44.990,0:00:47.720 we're also talking about other membrane-bound organelles 0:00:47.720,0:00:51.180 and in a close second place for a membrane-bound structure 0:00:51.180,0:00:52.720 that is very important to the cell 0:00:52.720,0:00:55.550 would be the mitochondria. 0:00:55.550,0:00:58.890 So let's draw some mitochondria right over here. 0:00:58.890,0:01:00.120 So I'll talk a little bit more about 0:01:00.120,0:01:02.020 what these little squiggly lines that I'm drawing 0:01:02.020,0:01:04.120 inside of the mitochondria are 0:01:04.120,0:01:05.290 and this is actually a little bit more 0:01:05.290,0:01:06.920 of a textbook visualization, as we'll learn 0:01:06.920,0:01:09.450 in a few minutes or seconds that we now have more 0:01:09.450,0:01:12.050 sophisticated visualizations of what's actually going on 0:01:12.050,0:01:13.690 inside of a mitochondria, but we haven't actually 0:01:13.690,0:01:15.250 answered all of our questions, 0:01:15.250,0:01:17.150 but you might have already learned that, 0:01:17.150,0:01:20.090 so let me make it clear, these are mitochondria. 0:01:20.090,0:01:21.020 That's the plural. 0:01:21.020,0:01:22.050 If we're just talking about one of them, 0:01:22.050,0:01:24.220 we're talking about a mitochondrion. 0:01:24.220,0:01:26.120 That's the singular of mitochondria. 0:01:26.120,0:01:27.720 But you might have already learned, 0:01:27.720,0:01:31.190 some time in your past or in another Khan Academy video, 0:01:31.190,0:01:35.960 that these are viewed as the ATP factories for cells. 0:01:35.960,0:01:38.750 So let me right it this way. 0:01:38.750,0:01:40.990 So ATP factories. 0:01:40.990,0:01:45.720 A-T-P factories and if you watched the videos on ATP 0:01:45.720,0:01:48.050 or cellular respiration or other videos, 0:01:48.050,0:01:51.720 I'd repeatedly talk about how ATP is really the currency 0:01:51.720,0:01:55.090 for energy in the cell that when it's in its ATP form 0:01:55.090,0:01:57.150 you have adenosine triphosphate. 0:01:57.150,0:02:00.150 If you pop one of the phosphate groups off, 0:02:00.150,0:02:02.920 you pop one of the P's off, it release energy 0:02:02.920,0:02:05.320 and that's what your body uses to do all sort of things 0:02:05.320,0:02:08.420 from movement to thinking to all sorts of things 0:02:08.420,0:02:10.450 that actually go on in your bodies, 0:02:10.450,0:02:12.850 so you can imagine mitochondria are really important 0:02:12.850,0:02:16.450 for energy, for when the cell has to do things. 0:02:16.450,0:02:18.760 And that's why you'll find more mitochondria 0:02:18.760,0:02:21.210 in things like muscle cells, things that have to use 0:02:21.210,0:02:22.950 a lot of energy. 0:02:22.950,0:02:25.990 Now before I get into the structure of mitochondria, 0:02:25.990,0:02:28.590 I wanna talk a little bit about its fascinating past 0:02:28.590,0:02:31.550 because we think of cells as the most basic unit of life 0:02:31.550,0:02:33.850 and that is true, that comes straight out of cell theory, 0:02:33.850,0:02:36.450 but it turns out the most prevalent theory 0:02:36.450,0:02:39.620 of how mitochondria got into our cells 0:02:39.620,0:02:43.390 is that at one time the predecessors, 0:02:43.390,0:02:45.220 the ancestors to our mitochondria, 0:02:45.220,0:02:49.790 were free, independent organisms, microorganisms. 0:02:49.790,0:02:54.320 So they're descendent from bacterial-like microorganisms 0:02:54.320,0:02:57.550 that might have been living on their own 0:02:57.550,0:02:59.650 and they were maybe really good at processing energy 0:02:59.650,0:03:01.290 or maybe they were even good at other things, 0:03:01.290,0:03:04.190 but at some point in the evolutionary past, 0:03:04.190,0:03:08.050 they got ingested by what the ancestors of our cells 0:03:08.050,0:03:10.510 and instead of just being engulfed and being torn to shreds 0:03:10.510,0:03:12.450 and kind of being digested and eaten, 0:03:12.450,0:03:15.250 it was like, "Hey, wait, if these things stick around, 0:03:15.250,0:03:17.120 "those cells are more likely to survive 0:03:17.120,0:03:19.850 "because they're able to help process glucose 0:03:19.850,0:03:24.550 "or help generate more energy out of things." 0:03:24.550,0:03:27.920 And so the cells that were able to kind of live 0:03:27.920,0:03:30.320 in symbiosis have them kind of give a place 0:03:30.320,0:03:33.190 for the mitochondria to live or the pre-mitochondria, 0:03:33.190,0:03:35.490 the ancestor mitochondria, those survived 0:03:35.490,0:03:38.990 and then through kind of the processes of natural selection, 0:03:38.990,0:03:41.350 this is what we now associate, 0:03:41.350,0:03:43.290 we now associate eukaryotic cells 0:03:43.290,0:03:44.890 as having mitochondria, 0:03:44.890,0:03:47.820 so I find this whole idea of one organism being inside 0:03:47.820,0:03:52.090 of another organism in symbiosis even at the cellular level, 0:03:52.090,0:03:55.350 that's kind of mind-boggling, but anyway, 0:03:55.350,0:03:57.590 I'll stop talking about that and now let's just talk 0:03:57.590,0:03:59.850 about the present, let's talk about 0:03:59.850,0:04:03.220 what the actual structure of mitochondria are. 0:04:03.220,0:04:06.120 And I'll first draw kind of a simplified drawing 0:04:06.120,0:04:10.050 of a mitochondion and I'll draw a cross section. 0:04:10.050,0:04:13.120 So, I'm gonna draw a cross section. 0:04:13.120,0:04:15.590 So if we were to kind of cut it in half. 0:04:15.590,0:04:16.990 So what I've drawn right over here 0:04:16.990,0:04:18.720 this would be its outer membrane. 0:04:18.720,0:04:24.190 This is the outer membrane right over here 0:04:24.190,0:04:25.520 and we label that. 0:04:25.520,0:04:28.390 Outer membrane. 0:04:28.390,0:04:30.588 And all of these membranes that I'm gonna draw, 0:04:30.588,0:04:34.250 they're all going to be phospholipid bilayers. 0:04:34.250,0:04:38.550 So if I were to zoom in right over here, 0:04:38.550,0:04:41.420 so let me, if I were to zoom in, 0:04:41.420,0:04:44.390 we would see a bilayer of phospholipids. 0:04:44.390,0:04:47.350 So you have your 0:04:47.350,0:04:50.490 hydrophilic heads facing outwards, 0:04:50.490,0:04:53.390 hydrophilic heads facing outwards 0:04:53.390,0:04:57.050 and your hydrophobic tails facing inwards. 0:04:57.050,0:04:59.200 So. 0:05:00.090,0:05:02.250 You see something just like that, 0:05:02.250,0:05:05.880 so they're all phospholipid bilayers. 0:05:05.880,0:05:07.890 But they aren't just phospholipids. 0:05:07.890,0:05:10.220 All of these membranes have all sorts of proteins imbedded, 0:05:10.220,0:05:13.250 I mean cells are incredibly complex structures, 0:05:13.250,0:05:16.890 but even organelles like mitochondria have a fascinating, 0:05:16.890,0:05:18.690 I guess you would say sub-structure to them. 0:05:18.690,0:05:21.120 They themselves have all sorts of interesting proteins, 0:05:21.120,0:05:23.150 enzymes imbedded in their membranes 0:05:23.150,0:05:25.450 and are able to help regulate what's going on 0:05:25.450,0:05:28.490 inside and outside of these organelles. 0:05:28.490,0:05:34.760 And one of the proteins that you have in the outer membrane 0:05:34.760,0:05:38.612 of mitochondria, they're called porins 0:05:38.612,0:05:40.390 and porins aren't found only in mitochondria, 0:05:40.390,0:05:41.590 but they're kind of tunnel proteins, 0:05:41.590,0:05:44.450 they're structured so they kind of form a hole 0:05:44.450,0:05:46.550 in the outer membrane. 0:05:46.550,0:05:49.350 So I'm drawing them the best that I can. 0:05:49.350,0:05:51.080 These are porins 0:05:51.080,0:05:54.320 and what's interesting about porins is they don't allow 0:05:54.320,0:05:57.120 large molecules to pass through passively, 0:05:57.120,0:06:02.050 but small molecules like sugars or ions can pass passively 0:06:02.050,0:06:04.090 through the porins. 0:06:04.090,0:06:06.120 And so, because of that, your ion concentration 0:06:06.120,0:06:07.450 and well, I should actually say, 0:06:07.450,0:06:09.085 your small molecule concentrations tend 0:06:09.085,0:06:12.555 to be similar on either side of this membrane, 0:06:12.555,0:06:15.085 on either side of this outer membrane. 0:06:15.085,0:06:17.125 But that's not the only membrane involved 0:06:17.125,0:06:18.845 in a mitochondrion. 0:06:18.845,0:06:20.685 We also have a inner membrane. 0:06:20.685,0:06:22.605 I'll do that in yellow. 0:06:22.955,0:06:24.155 We also have a inner membrane 0:06:24.155,0:06:27.285 and I'm gonna draw it with a textbook model first 0:06:27.285,0:06:28.545 and then we'll talk a little bit about, 0:06:28.545,0:06:32.885 since we think this model is not quite right, 0:06:32.885,0:06:36.685 but in this, so we have this inner membrane, 0:06:36.685,0:06:41.625 inner membrane, 0:06:41.625,0:06:45.525 and this inner membrane has these folds in it 0:06:45.525,0:06:46.985 to increase their surface area 0:06:46.985,0:06:48.555 and the surface area is really important 0:06:48.555,0:06:50.485 for the inner membrane because that's 0:06:50.485,0:06:53.355 where the processes of the electron transport chain 0:06:53.355,0:06:55.955 occur across, essentially, these membranes. 0:06:55.955,0:06:57.385 So you want this extra surface area 0:06:57.385,0:07:00.685 so you can essentially have more of that going on. 0:07:00.685,0:07:02.585 And these folds have a name. 0:07:02.585,0:07:04.225 So if you're talking about one of them, 0:07:04.225,0:07:05.725 if you're talking about one of these folds, 0:07:05.725,0:07:10.005 you're talking about a crista, 0:07:10.005,0:07:14.485 but if you're talking about more than one of them, 0:07:14.485,0:07:18.555 you would call that a cristae, cristae. 0:07:18.555,0:07:20.085 Sometimes I've seen the pronunciation of this 0:07:20.085,0:07:24.525 as cristae, cristae or cristae, that's plural for crista. 0:07:24.525,0:07:27.925 These are just folds in the inner membrane 0:07:27.925,0:07:30.125 and once again the inner membrane is also 0:07:30.125,0:07:33.655 a phospholipid bilayer. 0:07:33.655,0:07:36.085 Now inside of the inner membranes, 0:07:36.085,0:07:40.085 so between the outer membrane and the inner membrane 0:07:40.085,0:07:42.985 you could imagine what this is gonna be called. 0:07:42.985,0:07:46.485 That space is called the intermembrane space, 0:07:46.485,0:07:51.885 not too creative of a name, intermembrane space 0:07:51.885,0:07:53.885 and because of the porins, 0:07:53.885,0:07:56.545 the small molecule concentration 0:07:56.545,0:07:58.985 of the intermembrane space and then outside 0:07:58.985,0:08:02.685 of the mitochondria, 0:08:02.685,0:08:03.855 out in the cytosol, 0:08:03.855,0:08:05.825 those concentrations are gonna be similar, 0:08:05.825,0:08:08.585 but then the inner membrane does not have the porins 0:08:08.585,0:08:11.425 in it and so you can actually have a different concentration 0:08:11.425,0:08:12.655 on either side and that is essential 0:08:12.655,0:08:14.625 for the electron transport chain. 0:08:14.625,0:08:16.555 The electron transport chain really culminates 0:08:16.555,0:08:19.055 with hydrogen, a hydrogen ion gradient 0:08:19.055,0:08:20.785 being built between the two sides 0:08:20.785,0:08:24.085 and then they flow down that gradient through a protein 0:08:24.085,0:08:28.125 called ATP synthase which helps us synthesize ATP, 0:08:28.125,0:08:29.925 but we'll talk more about that maybe in this video 0:08:29.925,0:08:31.255 or in a future video, 0:08:31.255,0:08:33.255 but let's finish talking about the different parts 0:08:33.255,0:08:36.555 of a mitochondrion. 0:08:36.555,0:08:40.125 So inside the inner membrane you have 0:08:40.125,0:08:42.855 this area right over here is called the matrix. 0:08:42.855,0:08:45.685 It's called, let me use this in a different color, 0:08:45.685,0:08:47.875 this is 0:08:48.555,0:08:49.385 the matrix 0:08:49.385,0:08:50.955 and it's called the matrix 'cause it actually has 0:08:50.955,0:08:53.225 a much higher protein concentration, 0:08:53.225,0:08:58.085 it's actually more viscous than the cytosol 0:08:58.085,0:09:02.185 that would be outside of the 0:09:03.675,0:09:05.385 mitochondria. 0:09:05.385,0:09:07.525 So this right over here is the matrix. 0:09:07.525,0:09:09.815 When we we talk about cellular respiration, 0:09:09.815,0:09:11.925 cellular respiration has many phases in it. 0:09:11.925,0:09:13.345 We talk about glycolysis. 0:09:13.345,0:09:15.955 Glycolysis is actually occurring in the cytosol. 0:09:15.955,0:09:20.315 So glycolysis can occur in the cytosol. 0:09:20.315,0:09:22.855 Glycolysis. 0:09:22.855,0:09:26.025 But the other major phases of cellular respiration. 0:09:26.025,0:09:28.055 Remember we talk about the citric acid cycle 0:09:28.055,0:09:29.855 also known as the Krebs cycle, 0:09:29.855,0:09:32.585 that is occuring in the matrix. 0:09:32.585,0:09:35.825 So Krebs cycle 0:09:35.825,0:09:37.225 is occuring in the matrix 0:09:37.225,0:09:40.525 and then I said the electron transport chain 0:09:40.525,0:09:42.155 which is really what's responsible for producing 0:09:42.155,0:09:46.185 the bulk of the ATP, that is happening through proteins 0:09:46.185,0:09:49.255 that are straddling the inner membrane 0:09:49.255,0:09:55.125 or straddling the cristae right over here. 0:09:55.125,0:09:56.855 Now we're just done. 0:09:56.855,0:09:59.085 Probably one of the most fascinating parts of mitochondria, 0:09:59.085,0:10:01.985 we said that we think that they are descendent 0:10:01.985,0:10:04.725 from these ancient independent lifeforms 0:10:04.725,0:10:06.825 and in order to be an ancient independent life form, 0:10:06.825,0:10:09.055 they'd would have to have some information, 0:10:09.055,0:10:13.655 some way to actually transmit their genetic information 0:10:13.655,0:10:16.185 and, it turns out, mitochondria actually have 0:10:16.185,0:10:18.085 their own genetic information. 0:10:18.085,0:10:19.785 They have mitochondrial DNA 0:10:19.785,0:10:21.785 and they often don't just even have one copy of it, 0:10:21.785,0:10:23.415 they have multiple copies of it 0:10:23.415,0:10:28.225 and they're in loops very similar to bacterial DNA. 0:10:28.225,0:10:29.585 In fact, they have a lot in common 0:10:29.585,0:10:30.855 with bacterial DNA and that's why we think 0:10:30.855,0:10:34.385 that the ancestor to mitochondria that live independently 0:10:34.385,0:10:38.655 was probably a form of bacteria or related to bacteria 0:10:38.655,0:10:39.985 in some way. 0:10:39.985,0:10:41.485 So this is, this right over there, 0:10:41.485,0:10:43.885 that is the loop of mitochondrial DNA. 0:10:43.885,0:10:46.555 So all the DNA that's inside of you, the bulk of it, 0:10:46.555,0:10:50.385 yes, it is in your nuclear DNA, but you still have 0:10:50.385,0:10:52.625 a little bit of DNA in your mitochondria 0:10:52.625,0:10:55.615 and what's interesting is your mitochondrial DNA, 0:10:55.615,0:10:59.455 your mitochondria, are inherited, essentially, 0:10:59.455,0:11:03.275 from your mother's side, because when a egg is fertilized, 0:11:05.545,0:11:08.825 a human egg has tons of mitochondria in it 0:11:09.615,0:11:11.785 and I'm obviously not drawing all of the things 0:11:11.785,0:11:12.825 in the human egg. 0:11:12.825,0:11:14.285 It obviously has a nucleus and all of that. 0:11:14.285,0:11:16.355 The sperm has some mitochondria in it, 0:11:17.985,0:11:20.467 you could imagine it needs to be able 0:11:20.467,0:11:22.325 to win that very competitive fight 0:11:22.325,0:11:24.125 to get to fertilize the egg, 0:11:24.125,0:11:26.355 but the current theory is all or most of that 0:11:26.355,0:11:30.085 gets digested or dissolved once it actually gets 0:11:30.085,0:11:31.525 into the egg. 0:11:31.525,0:11:34.485 And anyway, the egg itself has way more mitochondria, 0:11:34.485,0:11:38.755 so the DNA in your mitochondria is 0:11:38.755,0:11:42.085 from your mother or is essentially from your mother's side 0:11:42.085,0:11:43.655 and that's actually used, mitochondrial DNA, 0:11:43.655,0:11:45.585 when people talk about kind of an ancient Eve 0:11:45.585,0:11:47.425 or tracing back to having kind 0:11:47.425,0:11:48.625 of one common mother, 0:11:48.625,0:11:52.885 people are looking at the mitochondrial DNA, 0:11:52.885,0:11:56.785 so it is actually quite, quite fascinating. 0:11:56.785,0:11:58.725 Now I said a little bit earlier, 0:11:58.725,0:12:00.855 and you know, obviously, it has its own DNA 0:12:00.855,0:12:02.345 and then because it has its own DNA 0:12:02.345,0:12:04.455 it's able to synthesize some of its own RNA, 0:12:04.455,0:12:07.555 its own ribosomes, so it also has ribosomes here. 0:12:07.555,0:12:10.455 But it doesn't synthesize all of the proteins 0:12:10.455,0:12:11.725 that are sitting in mitochondria. 0:12:11.725,0:12:14.385 A lot of those are still synthesized by 0:12:14.385,0:12:17.455 or encoded for by your nuclear DNA 0:12:17.455,0:12:19.955 and are actually synthesized outside of the mitochondria 0:12:19.955,0:12:22.885 and then make their way into the mitochondria, 0:12:22.885,0:12:25.855 but mitochondria are these fascinating, fascinating things. 0:12:25.855,0:12:29.455 They're these little creatures living in symbiosis 0:12:29.455,0:12:32.355 in our cells and they're able to replicate themselves 0:12:32.355,0:12:34.685 and I don't know, I find all of this mind boggling. 0:12:34.685,0:12:35.385 But anyway. 0:12:35.385,0:12:37.785 I said that this was the textbook model 0:12:37.785,0:12:39.725 because it turns out, when you look 0:12:39.725,0:12:43.685 at a micrograph, a picture of mitochondria, 0:12:43.685,0:12:45.485 it seems to back up this textbook model 0:12:45.485,0:12:49.125 of these folds, these cristae just kind of folding in, 0:12:49.125,0:12:51.755 but when we've been able to have more sophisticated 0:12:51.755,0:12:53.645 visualizations it actually turns out 0:12:53.645,0:12:55.985 that it's not just these simple folds 0:12:55.985,0:12:59.055 that the inner membrane essentially hooks 0:12:59.055,0:13:02.255 into the matrix and it turns out it has 0:13:02.255,0:13:05.085 these little tunnels that connect the space 0:13:05.085,0:13:10.525 inside of the cristae to the intermembrane space. 0:13:10.525,0:13:12.825 So I like to think about this because it makes you realize, 0:13:12.825,0:13:14.825 you know, we look in textbooks and we take these things 0:13:14.825,0:13:17.325 like mitochondria for granted, like, "Oh yeah, of course. 0:13:17.325,0:13:18.875 "That's where ATP factories are," 0:13:18.875,0:13:21.685 but it's still an area for visualization research 0:13:21.685,0:13:24.455 to fully understand exactly how they work 0:13:24.455,0:13:25.855 and even how they are structured 0:13:25.855,0:13:28.655 that this Baffle Model where you see these cristae 0:13:28.655,0:13:31.225 kind of just coming in and out of the different sides. 0:13:31.225,0:13:33.885 This is actually no longer the accepted model 0:13:33.885,0:13:36.745 for the actual visualization, the structure of mitochondria. 0:13:36.745,0:13:39.485 Something more like this, something more where 0:13:39.485,0:13:42.315 you have this cristae junction model 0:13:42.315,0:13:46.615 where you have, if I were to draw a cross section 0:13:46.615,0:13:50.025 where this is the, 0:13:50.025,0:13:52.255 I drew the outer membrane and the inner membrane, 0:13:52.255,0:13:54.655 I'll just draw has these little tunnels 0:13:54.655,0:13:59.535 to the actual space inside of the cristae. 0:13:59.535,0:14:03.455 This is actually now the more accepted visualization, 0:14:03.455,0:14:04.725 so I want you to appreciate 0:14:04.725,0:14:06.885 that when in Biology, you read something in a textbook 0:14:06.885,0:14:08.225 you kind of say, "Oh, people have figured all 0:14:08.225,0:14:10.285 "of this stuff out," but people are still think about, 0:14:10.285,0:14:11.625 "Well, how does this structure work? 0:14:11.625,0:14:12.785 "What is the actual structure?" and then, 0:14:12.785,0:14:14.825 "How does it actually let this organelle, 0:14:14.825,0:14:16.755 "this fascinating organelle do all of the things 0:14:16.755,0:14:18.805 "that it needs to do?"