WEBVTT 1 00:00:00.120 --> 00:00:03.200 Usually when we look at a skyscraper, we really just 2 00:00:03.279 --> 00:00:06.040 see the massive glass structure, you know, reflending the clouds. 3 00:00:06.040 --> 00:00:08.320 We just admire the whole thing at a glance, and 4 00:00:08.359 --> 00:00:11.839 we're completely ignoring the millions of individual bolts and the 5 00:00:11.880 --> 00:00:16.039 miles of hidden wiring and you know, the microscopic structural tolerances. 6 00:00:16.079 --> 00:00:19.920 They're basically keeping the entire edifice from just collapsing. Oh absolutely, 7 00:00:20.120 --> 00:00:23.320 And the human body operates on the exact same principle, 8 00:00:23.399 --> 00:00:26.120 Like we look in the mirror and we see a single, 9 00:00:26.480 --> 00:00:31.519 unified organism, but it is incredibly easy to overlook the 10 00:00:31.719 --> 00:00:38.119 trillions of highly engineered microscopic processes firing off every single 11 00:00:38.159 --> 00:00:40.600 millisecond just to keep us standing up right. 12 00:00:40.719 --> 00:00:43.280 It really is. Yeah, And the moment you start zooming in, 13 00:00:43.759 --> 00:00:45.920 you realize that the foundation of our health, and really 14 00:00:46.000 --> 00:00:49.320 the root of almost every disease lies in those invisible 15 00:00:49.399 --> 00:00:52.159 molecular nuts and bolts. We tend to think of medicine 16 00:00:52.200 --> 00:00:56.920 in macroscopic terms, you know, diagnosing a cough, treating a rash, 17 00:00:57.039 --> 00:01:01.039 or setting a broken bone. But true medical science requires 18 00:01:01.119 --> 00:01:05.879 understanding the machinery at the most fundamental cellular level. Because 19 00:01:06.040 --> 00:01:07.840 I mean, if you don't understand how the engine works. 20 00:01:07.879 --> 00:01:09.239 You can't truly fix the car. 21 00:01:09.400 --> 00:01:13.640 No, exactly. And that's actually the mission of today's deep dive. 22 00:01:14.200 --> 00:01:17.079 We are going to build the ultimate audio cheat sheet 23 00:01:17.120 --> 00:01:19.280 for how the body works, from the micro to the macro. 24 00:01:19.400 --> 00:01:21.640 I love it, and to do that, I was going 25 00:01:21.680 --> 00:01:25.519 through some of the foundational principles in this highly regarded textbook, 26 00:01:25.560 --> 00:01:28.959 the twenty twelve First Aid for the Basic Science's General Principles. 27 00:01:29.000 --> 00:01:30.920 Oh yeah, that's a classic, it really is. 28 00:01:31.079 --> 00:01:32.680 And I was just trying to wrap my head around 29 00:01:32.680 --> 00:01:35.599 how this machine actually operates, and I got stuck right 30 00:01:35.680 --> 00:01:38.200 at the very beginning, like at the very boundary of 31 00:01:38.239 --> 00:01:40.560 the cell itself, the plasma membrane. 32 00:01:40.640 --> 00:01:42.840 Ah, yes, the gatekeeper right. 33 00:01:42.920 --> 00:01:46.439 And chemically, the text describes it as an amphphilic lipid bilayer, 34 00:01:46.480 --> 00:01:48.480 which to me seems like a total contradiction. You have 35 00:01:48.680 --> 00:01:51.840 these molecules that somehow love water and hate water at 36 00:01:51.840 --> 00:01:52.799 the same exact time. 37 00:01:53.120 --> 00:01:56.719 It does sound paradoxical, but that dual personality is actually 38 00:01:56.760 --> 00:02:01.280 the only reason cellular life can exist at all. Amphophilic 39 00:02:01.400 --> 00:02:04.959 just means the molecule has two distinct regions. Okay, think 40 00:02:05.000 --> 00:02:06.319 of it like a match stick. The head of the 41 00:02:06.359 --> 00:02:09.360 match is a phosphate group that is hydrophilic, so it 42 00:02:09.400 --> 00:02:12.759 loves water, right, But the wooden stick part is made 43 00:02:12.800 --> 00:02:18.520 of fatty acid tails that are hydrophobic. They actively repel water. Now, 44 00:02:18.639 --> 00:02:21.400 because the environment inside the human body is mostly water 45 00:02:21.759 --> 00:02:24.800 and the fluid inside the cell is mostly water, these 46 00:02:24.840 --> 00:02:28.719 molecules they automatically arrange themselves into a two layered sheet, 47 00:02:29.159 --> 00:02:31.439 and they do it without needing any extra energy. 48 00:02:31.680 --> 00:02:34.680 Wait, they just self assemble, like automatically. 49 00:02:34.280 --> 00:02:38.360 Completely self assemble based purely on thermodynamics. Yeow Yeah. The 50 00:02:38.400 --> 00:02:41.400 water loving heads point outward towards the fluid unphal sides 51 00:02:41.680 --> 00:02:44.000 and the water faring pails they huddle together in the 52 00:02:44.039 --> 00:02:46.199 center of the sandwich, hiding from the moisture. 53 00:02:46.319 --> 00:02:48.479 Okay, that makes a lot of sense visually. 54 00:02:48.560 --> 00:02:52.879 Right, And this creates a highly selective barrier, so water 55 00:02:52.960 --> 00:02:56.000 and certain small molecules can slip through, but larger or 56 00:02:56.080 --> 00:02:59.840 charged molecules are totally blocked. It regulates the cell's entire 57 00:03:00.039 --> 00:03:00.800 ternal environment. 58 00:03:00.879 --> 00:03:03.199 So once we get past that selective barrier, we are 59 00:03:03.400 --> 00:03:06.759 inside the cell. It's essentially a microscopic city at this point. Definitely, 60 00:03:06.800 --> 00:03:09.400 we have the nucleus which holds the DNA, but I 61 00:03:09.439 --> 00:03:12.919 was really surprised to learn there is a structure inside 62 00:03:12.919 --> 00:03:17.039 the nucleus, the nucleolus, that doesn't deal with DNA directly, 63 00:03:17.080 --> 00:03:20.159 but rather it produces ribosomal RNA. 64 00:03:19.919 --> 00:03:24.520 Yes, the nucleolus, and that ribosomal RNA or r RNA 65 00:03:24.960 --> 00:03:28.360 is crucial because it forms the physical structure of ribivomes. 66 00:03:28.680 --> 00:03:31.120 And you can think of ribosomes as basically microscopic three 67 00:03:31.159 --> 00:03:33.840 D printers. Okay, so once they're built in the nucleolis, 68 00:03:33.840 --> 00:03:36.680 they leave the nucleus and they attach themselves to the 69 00:03:36.759 --> 00:03:41.840 sprawling membrane network called the endoplasmic reticulum, specifically the rough er. 70 00:03:42.080 --> 00:03:44.599 Oh. And it's called rough because under microscope it looks 71 00:03:44.599 --> 00:03:47.039 studded and bumpy with all those ribosomes. 72 00:03:46.599 --> 00:03:49.479 Right exactly, it looks incredibly bumpy. And this is the 73 00:03:49.520 --> 00:03:53.439 cell's primary protein factory. Cells that need to secrete massive 74 00:03:53.439 --> 00:03:56.560 amounts of protein, like say the pancreatic cells, the pump 75 00:03:56.599 --> 00:04:01.039 out digestive enzymes, they have these heavily expanded rough er networks. 76 00:04:01.319 --> 00:04:03.439 Okay, so if the cell is a city, the rough 77 00:04:03.719 --> 00:04:05.639 R is the industrial manufacturing district. 78 00:04:05.680 --> 00:04:06.599 That's a perfect analogy. 79 00:04:06.680 --> 00:04:08.800 Yeah, what about the parts of the er that aren't 80 00:04:09.080 --> 00:04:12.080 studded with ribosomes, the smooth sections. 81 00:04:12.400 --> 00:04:16.600 The smooth er handles an entirely different manufacturing process. So 82 00:04:16.680 --> 00:04:20.879 it synthesizes lipids, fatty acids, and steroid hormones. Oh okay, 83 00:04:20.959 --> 00:04:24.800 and it also plays a massive role in detoxifying chemicals. So, 84 00:04:24.959 --> 00:04:26.519 for example, if you were to look at the litter 85 00:04:26.560 --> 00:04:30.720 cells the patasites of someone who regularly consumes alcohol or 86 00:04:30.720 --> 00:04:35.000 takes certain medications, you would actually see a highly hypertro feed, 87 00:04:35.079 --> 00:04:39.360 heavily reinforced smooth r working overtime just to clear those 88 00:04:39.360 --> 00:04:40.519 toxins from the bloodstream. 89 00:04:40.680 --> 00:04:43.120 Man, the body is so adaptive. Okay. So the cell 90 00:04:43.160 --> 00:04:45.720 is churning out proteins and lipids on this factory floor, 91 00:04:45.920 --> 00:04:48.680 but they can't just float around randomly, right, They need 92 00:04:48.720 --> 00:04:51.480 to be processed and shipped out, which brings us to 93 00:04:51.879 --> 00:04:53.319 the gold gee apparatus. 94 00:04:53.439 --> 00:04:54.319 Yes, the golgie. 95 00:04:54.480 --> 00:04:58.279 I keep visualizing this as like a biological post office, 96 00:04:58.319 --> 00:05:01.000 taking these newly minted proteins, telling them where to go. 97 00:05:01.319 --> 00:05:04.000 That is honestly the most accurate way to conceptualize it. 98 00:05:04.519 --> 00:05:08.360 The gul gie receives raw proteins on its receiving doc 99 00:05:08.480 --> 00:05:11.480 which is called the cyspace, and it modifies them as 100 00:05:11.560 --> 00:05:14.439 they move through its internal networks, and that it ships 101 00:05:14.439 --> 00:05:17.120 them out in vesicles from the transface. But one of 102 00:05:17.160 --> 00:05:20.639 the most vital modifications it performs is attaching a specific 103 00:05:20.720 --> 00:05:26.720 molecular tag called MANO six phosphate to certain proteins manix phosphate. Right, 104 00:05:26.879 --> 00:05:29.879 That tag acts as a literal zip code. It tells 105 00:05:29.920 --> 00:05:33.800 the transport vesicle to deliver that specific protein directly to 106 00:05:33.879 --> 00:05:34.519 the lysisome. 107 00:05:34.639 --> 00:05:37.600 And the lysism is basically the cells recycling center, right, 108 00:05:37.720 --> 00:05:39.240 it breaks down the cellular trash. 109 00:05:39.360 --> 00:05:43.120 It is, and it's a super extreme environment. Lysisoms maintain 110 00:05:43.240 --> 00:05:45.560 an internal pH below four. 111 00:05:45.399 --> 00:05:48.240 Point eight, which is highly acidic, very acidic, and. 112 00:05:48.199 --> 00:05:52.399 They achieve this by constantly burning atp to pump hydrogen 113 00:05:52.439 --> 00:05:55.680 ions against their concentration gradient, just forcing them inside. Yeah, 114 00:05:55.720 --> 00:05:59.800 that extreme acidity is totally necessary to activate the hydrolytic 115 00:05:59.879 --> 00:06:04.480 enzymes that dissolve all that cellular waste, you know, old proteins, 116 00:06:04.519 --> 00:06:06.079 engulfed bacteria, all of it. 117 00:06:06.120 --> 00:06:08.399 Okay, let me pose a hypothetical here, because this is 118 00:06:08.399 --> 00:06:11.639 where the sheer fragility of the system kind of terrifies me. 119 00:06:11.759 --> 00:06:12.560 Okay, go for it. 120 00:06:12.600 --> 00:06:15.839 What happens if that golgie post office makes a clerical error, 121 00:06:16.199 --> 00:06:19.120 like what if it forgets to stamp that MANOS six 122 00:06:19.120 --> 00:06:21.800 phosphate ZIP code onto the recycling enzymes. 123 00:06:21.879 --> 00:06:25.879 Well, the clinical consequences are pretty devastating, honestly. What's fascinating 124 00:06:25.920 --> 00:06:28.360 here is that when the golgie fails to add that 125 00:06:28.439 --> 00:06:32.399 Manno six phosphate tag, the recycling enzymes never actually get 126 00:06:32.439 --> 00:06:35.439 delivered to the lysisom oh man and without a destination, 127 00:06:35.800 --> 00:06:39.040 the cell's default mechanism is just to secrete them outside 128 00:06:39.040 --> 00:06:40.319 the cell entirely. 129 00:06:40.360 --> 00:06:43.240 So the recycling enzymes just get dumped into the bloodstream 130 00:06:43.560 --> 00:06:46.519 and the lysism itself is left empty handed exactly. 131 00:06:46.959 --> 00:06:50.160 And because the lysism has no enzymes to break down, 132 00:06:50.199 --> 00:06:54.439 waste cellular garbage begins to accumulate inside the cell as 133 00:06:54.480 --> 00:06:55.920 these dense inclusion bodies. 134 00:06:56.000 --> 00:06:57.519 That sounds terrible it is. 135 00:06:57.839 --> 00:07:00.519 This is known as I cell disease. This is really 136 00:07:00.519 --> 00:07:04.959 severe developmental delays, coarse facial features and restricted joint movements, 137 00:07:05.600 --> 00:07:09.920 and tragically it usually proves fatal in early childhood. Oh wow. Yeah. 138 00:07:09.959 --> 00:07:13.240 It is a profound example of how a microscopic sorting 139 00:07:13.360 --> 00:07:17.759 error literally forgetting one single molecular tag cascades into a 140 00:07:17.759 --> 00:07:19.160 systemic fatal. 141 00:07:18.839 --> 00:07:22.600 Disease that is heavy And the text also mentioned tasas 142 00:07:22.600 --> 00:07:23.959 disease in a similar vein. Right. 143 00:07:24.000 --> 00:07:27.160 Yeah, TTAX is another lysosomal storage disease. Yeah, but in 144 00:07:27.199 --> 00:07:29.959 that case, instead of a missing TAG, the body is 145 00:07:30.040 --> 00:07:34.319 just genetically missing one specific lysosomal enzyme, hexysem in a 146 00:07:34.399 --> 00:07:37.720 ase a. So a specific fatty substance just builds up 147 00:07:37.720 --> 00:07:39.920 to toxic levels, mostly in the brain. 148 00:07:40.160 --> 00:07:42.319 It really puts into perspective how much we take for 149 00:07:42.439 --> 00:07:45.839 granted when things are working correctly. Absolutely, which makes me 150 00:07:45.920 --> 00:07:48.120 wonder about the logistics of all this, because if we 151 00:07:48.240 --> 00:07:50.920 have trillions of these cell cities and they all require 152 00:07:51.000 --> 00:07:54.279 constant energy, you know, like the mitochondria with its folded 153 00:07:54.279 --> 00:07:59.000 inner membranes, the criste just burning oxygen to make atp right, 154 00:07:59.160 --> 00:08:01.800 the supply by lines to keep them fed and defended, 155 00:08:01.879 --> 00:08:03.079 must be unimaginable. 156 00:08:03.439 --> 00:08:08.079 Unimaginable is the right word, and that logistics network is 157 00:08:08.120 --> 00:08:13.120 our cardiovascular system. The vehicles are the blood cells, and 158 00:08:13.319 --> 00:08:16.240 every single blood cell in your body, whether it's carrying 159 00:08:16.279 --> 00:08:19.720 oxygen or fighting a virus, originates from one single place, 160 00:08:19.959 --> 00:08:24.160 the bone marrow. Specifically, they come from pluripotent stem cells. 161 00:08:24.519 --> 00:08:27.920 These are basically the master architects of the blood, and 162 00:08:28.240 --> 00:08:32.000 they utilize a really fascinating mechanism called asymmetrical reproduction. 163 00:08:32.240 --> 00:08:35.200 Asymmetrical so meaning when the cell divides into two, the 164 00:08:35.279 --> 00:08:36.399 daughter cells aren't identical. 165 00:08:36.519 --> 00:08:39.720 Correct. When a pluripotent stem cell divides, one of the 166 00:08:39.759 --> 00:08:42.480 new cells remains a stem cell and it just stays 167 00:08:42.519 --> 00:08:45.919 behind to maintain the marrow's permanent reserve. Oh, I see, 168 00:08:46.159 --> 00:08:49.200 But the other cell commits to differentiating into a specialized 169 00:08:49.200 --> 00:08:51.399 blood cell, and it will go down one of two 170 00:08:51.399 --> 00:08:54.519 major lineages, either the myoloid line or the lymphoid line. 171 00:08:54.600 --> 00:08:57.080 Let's follow the myloid line first, because that leads to 172 00:08:57.120 --> 00:09:00.000 the most abundance cell in the blood, right, the erythrocyte, 173 00:08:59.840 --> 00:09:02.360 the red blood cell. Yes, And they have that iconic 174 00:09:02.440 --> 00:09:05.320 biconcave shape like a doughnut where the hole doesn't go 175 00:09:05.360 --> 00:09:08.120 all the way through right exactly, And I know that 176 00:09:08.200 --> 00:09:10.919 shape increases their surface areas so they can grab onto 177 00:09:10.919 --> 00:09:14.039 as much oxygen as possible. But I have a fundamental 178 00:09:14.080 --> 00:09:15.399 logic question about this. 179 00:09:15.720 --> 00:09:16.320 Lay it on me. 180 00:09:17.519 --> 00:09:21.159 If their entire existence is dedicated to carrying oxygen, don't 181 00:09:21.200 --> 00:09:23.919 they need energy to actually swim through the bloodstream. Aren't 182 00:09:23.919 --> 00:09:26.000 they just burning up their own cargo before it even 183 00:09:26.039 --> 00:09:27.000 reaches the tissue. 184 00:09:27.120 --> 00:09:29.759 See that is a great question, and it's a brilliant 185 00:09:29.840 --> 00:09:34.480 evolutionary workaround. Red blood cells don't consume their cargo because 186 00:09:34.480 --> 00:09:35.519 they physically can't. 187 00:09:35.600 --> 00:09:35.840 Wait. 188 00:09:35.919 --> 00:09:39.200 Why not? Right before a mature red blood cell enters 189 00:09:39.240 --> 00:09:43.240 the bloodstream, it actively injects its nucleus in all of 190 00:09:43.240 --> 00:09:45.600 its organelles, including its mitochondria. 191 00:09:45.720 --> 00:09:47.759 It throws out its own power plants, It gets. 192 00:09:47.639 --> 00:09:49.279 Rid of them entirely. It just has one hundred and 193 00:09:49.279 --> 00:09:52.559 twenty day life span after that. Mitochondria are the only 194 00:09:52.679 --> 00:09:57.559 organelles capable of aerobic respiration, which means using oxygen for energy. Okay, 195 00:09:57.799 --> 00:10:01.360 so because red blood cells lack mitochondria, they are forced 196 00:10:01.399 --> 00:10:05.320 to rely on anaerobic glycolysis for their energy, which doesn't 197 00:10:05.399 --> 00:10:09.639 use oxygen at all. Yeah. So, by intentionally crippling their 198 00:10:09.639 --> 00:10:13.120 own metabolic machinery, they ensure that one hundred percent of 199 00:10:13.200 --> 00:10:16.000 the oxygen they carry is delivered to the oxygen star 200 00:10:16.039 --> 00:10:17.320 of tissues that actually need it. 201 00:10:17.600 --> 00:10:21.679 That is incredible. They are essentially delivery trucks with engines 202 00:10:21.720 --> 00:10:24.480 that run on a completely different fuel than the cargo 203 00:10:24.559 --> 00:10:27.600 they haul exactly. Okay, So alongside the red blood cells, 204 00:10:27.600 --> 00:10:30.559 we have the white blood cells, the leukocytes. These are 205 00:10:30.600 --> 00:10:34.399 the immune defenders, starting with the neutrophils. Yes, the nutril 206 00:10:35.000 --> 00:10:38.240 I understand they are the first responders to like an 207 00:10:38.279 --> 00:10:42.679 acute bacterial infection or inflammation, but the mechanism they use 208 00:10:42.759 --> 00:10:45.039 to actually kill the bacteria is wild. 209 00:10:45.120 --> 00:10:45.879 Oh it really is. 210 00:10:46.279 --> 00:10:46.360 So. 211 00:10:46.480 --> 00:10:49.720 Neutrophils are the foot soldiers of the innate immune system. 212 00:10:50.200 --> 00:10:53.279 When they detect a microbe, they phagocytocin, which just means 213 00:10:53.320 --> 00:10:56.320 they swallow it whole. But trapping the bacteria isn't enough. 214 00:10:56.360 --> 00:10:59.279 You have to destroy it. So neutrophils use an enzyme 215 00:10:59.320 --> 00:11:04.279 called inadh oxidase to pull oxygen from the surrounding tissue 216 00:11:04.720 --> 00:11:08.720 and create a massive oxidated burst of superoxide radicals. And then, 217 00:11:09.240 --> 00:11:13.679 using another system involving hydrogen, peroxide, and chloride, they synthesize 218 00:11:13.720 --> 00:11:16.840 hypochlorous acid directly inside the fagosome. 219 00:11:17.000 --> 00:11:20.240 Wait, hypochlorous acid isn't that literally household bleach? 220 00:11:20.480 --> 00:11:26.039 Chemically, yes, your neutrophils manufacture microscopic amounts of bleach to 221 00:11:26.200 --> 00:11:28.600 chemically burn the engulfed bacteria to death. 222 00:11:28.879 --> 00:11:30.759 That is terrifying and amazing. 223 00:11:30.960 --> 00:11:34.519 It is highly effective, but obviously having a bleach factory 224 00:11:34.600 --> 00:11:38.600 inside your bloodstream is incredibly dangerous. That's why the reaction 225 00:11:38.679 --> 00:11:43.200 is strictly contained within a tiny, heavily armored vacuuole inside 226 00:11:43.240 --> 00:11:43.799 the neutrophil. 227 00:11:43.879 --> 00:11:45.480 I will never think of a scrape me the same 228 00:11:45.480 --> 00:11:48.399 way again, honestly, right. And we also have other specialized 229 00:11:48.399 --> 00:11:51.759 defenders in this myloid lineage, right, like the asinophils for 230 00:11:51.799 --> 00:11:54.679 fighting off parasites. Yes, and then basophyls and mass cells, 231 00:11:54.720 --> 00:11:57.279 which I know drive me crazy every single spring because 232 00:11:57.279 --> 00:11:59.080 they mediate allergic responses. 233 00:11:59.279 --> 00:12:04.080 Yes. Chanism of allergies is largely driven by these cells degranulating. 234 00:12:04.440 --> 00:12:06.879 So mass cells are stationed out in your connective tissues 235 00:12:07.120 --> 00:12:10.360 while basophylls circulating the blood. And when a tissue resident 236 00:12:10.480 --> 00:12:14.720 mass cell encounters an allergen, it bursts open, releasing massive 237 00:12:14.720 --> 00:12:16.279 amounts of histamine and serotonin. 238 00:12:16.320 --> 00:12:19.360 Oh wait, mass cells half serotonin? I thought that was 239 00:12:19.399 --> 00:12:20.159 just a brain chemical. 240 00:12:20.360 --> 00:12:24.000 Yeah, mass cells contain serotonin, but interestingly basophylls do not. 241 00:12:24.399 --> 00:12:24.960 Ah. 242 00:12:25.039 --> 00:12:27.759 But anyway, the release of these chemicals causes the surrounding 243 00:12:27.759 --> 00:12:31.120 blood vessels to rapidly dilate and leak fluid, which is 244 00:12:31.120 --> 00:12:33.679 why your tissue swells up, your nose runs, and your 245 00:12:33.679 --> 00:12:34.559 skin gets all red. 246 00:12:34.679 --> 00:12:37.559 Niche good to know who to blame for my seasonal allergies. 247 00:12:38.080 --> 00:12:41.240 So that covers the innate immune system, the immediate sort 248 00:12:41.279 --> 00:12:45.120 of scorched earth defense. But what about the learned defense? 249 00:12:45.279 --> 00:12:48.039 You know, the cells that actually remember a virus years 250 00:12:48.159 --> 00:12:50.679 later that comes from the lymphoid lineage. 251 00:12:50.759 --> 00:12:54.080 Yes, the lymphocytes, the B cells and T cells, And 252 00:12:54.080 --> 00:12:56.360 the really simple way to remember their origins is that 253 00:12:56.399 --> 00:12:59.039 B cells develop in the bone marrow and T cells 254 00:12:59.080 --> 00:13:00.639 migrate to mature thymus. 255 00:13:00.679 --> 00:13:02.480 Oh B for bone, T for thymus. 256 00:13:02.519 --> 00:13:03.639 Easy, exactly. 257 00:13:03.799 --> 00:13:04.039 Yeah. 258 00:13:04.080 --> 00:13:06.879 So B cells handle humoral immunity, which means they act 259 00:13:06.919 --> 00:13:10.279 as the body's long range artillery. When they are activated 260 00:13:10.360 --> 00:13:13.840 by a pathogen, they transform into plasma cells and they 261 00:13:13.879 --> 00:13:19.159 begin manufacturing highly specific antibodies. And these antibodies just circulate 262 00:13:19.159 --> 00:13:22.919 through the blood, tagging and neutralizing invaders from a distance. 263 00:13:23.080 --> 00:13:26.039 And the T cells handle cellular immunity, so they're more 264 00:13:26.080 --> 00:13:29.519 like the close quarters infantry. They actually engage in cell 265 00:13:29.519 --> 00:13:32.360 to cell combat, scanning the surfaces of our own cells 266 00:13:32.399 --> 00:13:34.559 to see if they've been infected from the inside. Yes, 267 00:13:34.840 --> 00:13:37.000 but I was reading about the training process these T 268 00:13:37.120 --> 00:13:39.559 cells go through in the thymus and it sounds brutal. 269 00:13:39.720 --> 00:13:44.159 Oh, it is a ruthless quality control process because as 270 00:13:44.159 --> 00:13:47.240 T cells mature in the thymus, they are constantly presented 271 00:13:47.279 --> 00:13:50.559 with bits of our own healthy tissue are self antigens, okay, 272 00:13:50.799 --> 00:13:54.120 And if a developing T cells receptor accidentally reacts to 273 00:13:54.159 --> 00:13:57.559 our own tissue, the thymus forces that T cell to 274 00:13:57.639 --> 00:13:58.679 undergo apoptosis. 275 00:13:58.679 --> 00:14:00.240 It just kills it. 276 00:14:00.000 --> 00:14:03.480 Briggers its own program cell death. Because if this screening 277 00:14:03.519 --> 00:14:07.120 process fails and a self reactive T cell escapes into 278 00:14:07.159 --> 00:14:10.639 the blood stream, it will start attacking the body's own organs. 279 00:14:10.360 --> 00:14:14.399 Which is the underlying mechanism of autoimmune diseases exactly. Okay, 280 00:14:14.440 --> 00:14:18.360 so we've covered cellular factories, the supply trucks, the bleach 281 00:14:18.360 --> 00:14:22.480 wielding infantry. But how does this immense coordination happen? Like, 282 00:14:22.519 --> 00:14:26.440 who writes the instructions? Where is the master blueprint that 283 00:14:26.559 --> 00:14:30.720 codes for a B cell antibody or a lysism's proton pump. 284 00:14:31.600 --> 00:14:33.840 This brings us down to the atomic level, right, the 285 00:14:33.879 --> 00:14:35.720 biochemistry of DNA. 286 00:14:35.919 --> 00:14:38.360 Yes, the blueprint of life and it relies on an 287 00:14:38.360 --> 00:14:42.039 astonishingly simple alphabet. So DNA is composed of nucletides, and 288 00:14:42.080 --> 00:14:45.159 each nucleotide has a pentose sugar, a phosphate, and an 289 00:14:45.200 --> 00:14:48.120 etrogenous base. And there are only four bases, divided into 290 00:14:48.120 --> 00:14:50.000 two categories, purines and pyramidines. 291 00:14:50.120 --> 00:14:51.799 Right, and there's a mnemonic for this, isn't there? 292 00:14:51.840 --> 00:14:54.440 There is a helpful numonic used in medical training. Is 293 00:14:54.480 --> 00:14:57.360 purines are pure as gold. So the ANG stand for 294 00:14:57.480 --> 00:15:00.960 adnine and guanine and the other two s and thymine 295 00:15:01.120 --> 00:15:02.039 are the pyramidines. 296 00:15:02.279 --> 00:15:05.360 Okay, So AT nine pairs with thymine and guanine pairs 297 00:15:05.360 --> 00:15:08.960 with cytosine. That's the classic double helix, and that feeds 298 00:15:09.000 --> 00:15:12.879 into the central dogma of biology. Right, DNA is transcribed 299 00:15:12.879 --> 00:15:15.120 into RNA, which is translated into protein. 300 00:15:15.399 --> 00:15:15.879 You got it. 301 00:15:16.120 --> 00:15:19.399 But the bonds holding those DNA based pairs together aren't identical. 302 00:15:19.639 --> 00:15:23.240 Adenine and thymine connect using two hydrogen bonds, while guany 303 00:15:23.240 --> 00:15:26.960 and cytosine used three. So why does one extra microscopic 304 00:15:27.080 --> 00:15:29.320 bond actually matter in the real world? 305 00:15:29.600 --> 00:15:32.600 It matters immenally when you consider the physical stability of 306 00:15:32.639 --> 00:15:36.600 the DNA strand because guanine and cytosines share three hydrogen bonds, 307 00:15:36.720 --> 00:15:39.399 they hold onto each other much tighter. Okay, so if 308 00:15:39.440 --> 00:15:41.399 you have a sequence of DNA that is rich in 309 00:15:41.480 --> 00:15:44.840 G and C, it requires a significantly higher temperature to 310 00:15:44.879 --> 00:15:48.799 physically melt or dnature the two strands apart compared to 311 00:15:48.840 --> 00:15:50.120 a sequence heavy in A and T. 312 00:15:50.639 --> 00:15:51.200 Oh wow. 313 00:15:51.240 --> 00:15:53.840 And researchers use this principle every day in the lab. 314 00:15:54.080 --> 00:15:57.279 When they perform a polymerase chain reaction or PCR to 315 00:15:57.399 --> 00:16:00.799 copy DNA, they have to calculate the exact temperature needed 316 00:16:00.840 --> 00:16:03.399 to melt the patient's specific DNA sample based on that 317 00:16:03.480 --> 00:16:04.320 GC content. 318 00:16:04.639 --> 00:16:07.799 Here's where it gets really interesting to me. What happens 319 00:16:07.879 --> 00:16:12.440 when the blueprint gets corrupted mutations. I always just picture 320 00:16:12.519 --> 00:16:17.440 genetic mutations as random, chaotic typos, but there's a strict 321 00:16:17.559 --> 00:16:21.159 chemical hierarchy to them, right, like a transition mutation versus 322 00:16:21.200 --> 00:16:22.440 a transversion mutation. 323 00:16:22.639 --> 00:16:24.799 Yeah, and it all comes back to the physical shape 324 00:16:24.799 --> 00:16:28.279 of the molecules. So curines have a double ring chemical structure, 325 00:16:28.639 --> 00:16:32.159 while pure meetings have a single RINGKA. A transition mutation 326 00:16:32.279 --> 00:16:34.919 is a subtle swap. It's when a double ring curinge 327 00:16:34.960 --> 00:16:38.279 is accidentally replaced by the other double ring purine, So 328 00:16:38.399 --> 00:16:40.600 like an AD nine swapped for a guanine. 329 00:16:40.679 --> 00:16:42.840 Right, So the size is the same exactly. 330 00:16:43.000 --> 00:16:46.960 Yeah, the physical spacing of the DNA helix stays relatively similar. 331 00:16:47.440 --> 00:16:51.279 But a transversion mutation is a major structural disruption. That's 332 00:16:51.279 --> 00:16:53.960 when a double ring purine is erroneously replaced by a 333 00:16:54.000 --> 00:16:55.200 single ring pyramity. 334 00:16:55.360 --> 00:16:56.080 Oh I see. 335 00:16:56.240 --> 00:16:59.360 Yeah, so the physical architecture of the DNA helix actually warps, 336 00:16:59.559 --> 00:17:02.159 which makes it much harder for the cells repair machinery 337 00:17:02.159 --> 00:17:02.759 to overlook. 338 00:17:03.000 --> 00:17:06.200 That makes total sense. So the DNA holds these instructions 339 00:17:06.279 --> 00:17:09.759 and through transcription and translation, it builds proteins, and many 340 00:17:09.799 --> 00:17:12.240 of those proteins act as enzymes, which are the biological 341 00:17:12.240 --> 00:17:15.599 machines that actually make life happen. But I struggle to 342 00:17:15.680 --> 00:17:19.599 visualize exactly what an enzyme is doing dynamically. The literature 343 00:17:19.640 --> 00:17:22.880 always brings up the mchaeless menton model and talks about 344 00:17:22.960 --> 00:17:27.319 lowering the activation energy without changing the overall Gibbs free energy. 345 00:17:27.880 --> 00:17:29.200 Can you give me a visual for that? 346 00:17:29.599 --> 00:17:33.559 If we connect this to the bigger picture, Imagine you're 347 00:17:33.599 --> 00:17:36.720 standing at the base of a massive mountain and you 348 00:17:36.799 --> 00:17:39.480 need to push a heavy boulder to a valley on 349 00:17:39.519 --> 00:17:39.960 the other side. 350 00:17:40.039 --> 00:17:40.720 Okay, I'm with you. 351 00:17:41.240 --> 00:17:44.839 The final destination the valley is lower than your starting point, 352 00:17:44.960 --> 00:17:47.519 so mathematically the boulder wants to end up there. That 353 00:17:47.680 --> 00:17:50.480 difference in elevation between start and finish is your Gibs 354 00:17:50.480 --> 00:17:54.599 free energy. It dictates the final outcome. However, to get 355 00:17:54.640 --> 00:17:56.880 it to the valley, you first have to push that 356 00:17:56.960 --> 00:17:59.200 boulder all the way up and over the peak of 357 00:17:59.240 --> 00:18:03.319 the mountain. That massive initial effort is the activation energy. 358 00:18:03.440 --> 00:18:05.279 Without help, it might take you one hundred years to 359 00:18:05.279 --> 00:18:05.720 push it. 360 00:18:05.680 --> 00:18:08.839 Over, which is obviously too slow to keep a human 361 00:18:08.880 --> 00:18:10.160 body alive exactly. 362 00:18:10.440 --> 00:18:14.279 Biological reactions would take far too long. So an enzyme 363 00:18:14.319 --> 00:18:16.559 acts as a tunnel drilled straight through the center of 364 00:18:16.599 --> 00:18:19.680 the mountain. Oh nice, It doesn't change your starting point 365 00:18:19.799 --> 00:18:22.079 and it doesn't change your final destination in the valley. 366 00:18:22.119 --> 00:18:25.680 So the Gibbs free energy remains identical, but it completely 367 00:18:25.720 --> 00:18:30.839 bypasses the mountain peak. It drastically lowers the activation energy required, 368 00:18:31.079 --> 00:18:35.119 allowing that boulder to reach the valley in milliseconds instead 369 00:18:35.160 --> 00:18:35.839 of centuries. 370 00:18:35.920 --> 00:18:39.400 And because these tunnels are made of protein, they are 371 00:18:39.559 --> 00:18:43.559 highly sensitive to their environment right specifically heat. So is 372 00:18:43.640 --> 00:18:48.200 this why a severe fever feels so devastating, Like if 373 00:18:48.200 --> 00:18:50.720 my body temperature hits one hundred or four degrees, are 374 00:18:50.720 --> 00:18:53.039 my mountain tunnels literally collapsing? 375 00:18:53.359 --> 00:18:56.039 That is precisely what is happening up to a certain point. 376 00:18:56.079 --> 00:18:58.920 A little bit of heat increases kinetic energy, right, the 377 00:18:58.960 --> 00:19:02.680 molecules bounce around and the enzymes work quicker, But proteins 378 00:19:02.680 --> 00:19:05.880 are held together by fragile bonds. When the temperature gets 379 00:19:05.920 --> 00:19:08.440 too high, the three D structure of the enzyme begins 380 00:19:08.440 --> 00:19:12.640 to unravel. It denatures, the tunnel collapses, the chemical reactions 381 00:19:12.720 --> 00:19:15.960 grind to a halt, and cellular function just begins to 382 00:19:16.160 --> 00:19:17.319 fail systemically. 383 00:19:17.640 --> 00:19:20.680 Wow. Okay, So we have spent this entire time zooming 384 00:19:20.720 --> 00:19:25.200 in looking at molecular tunnels, DNA typos, and microscopic bleach factories. 385 00:19:25.880 --> 00:19:28.400 But medical science isn't just about what happens in a 386 00:19:28.440 --> 00:19:32.240 test tube. How do researchers take these microscopic mechanisms and 387 00:19:32.359 --> 00:19:35.559 track their impact across millions of people. We have to 388 00:19:35.640 --> 00:19:38.279 zoom all the way out to epidemiology, right we do. 389 00:19:38.799 --> 00:19:42.119 Epidemiology is how we observe these cellular interactions at a 390 00:19:42.160 --> 00:19:46.319 population scale. It's the mathematical foundation of Evening's based medicine, 391 00:19:46.599 --> 00:19:48.599 and two of the most crucial tools we use are 392 00:19:48.640 --> 00:19:51.680 cohort studies and case control studies, and they actually approach 393 00:19:51.759 --> 00:19:53.240 data from opposite directions. 394 00:19:53.839 --> 00:19:56.160 Yeah, I was reading about these two study designs and 395 00:19:56.200 --> 00:19:58.599 initially they sounded identical to me. You take a group 396 00:19:58.640 --> 00:20:00.359 of people, you look at a disease, and you look 397 00:20:00.359 --> 00:20:02.960 at a risk factor. What is the actual difference? 398 00:20:03.400 --> 00:20:07.599 The distinction lies in the timeline. So a cohort study 399 00:20:07.640 --> 00:20:12.200 looks forward. You assemble a group of healthy people a cohort, 400 00:20:12.279 --> 00:20:15.279 and you divide them based on an exposure. Okay, like 401 00:20:15.359 --> 00:20:18.039