0:00 In this video, we will look at lipoproteins. 0:10 Lipoproteins are basically fat lipo and proteins. 0:17 And there are different types of lipoproteins. 0:20 You can say that there are four types. 0:22 These are collie microns. 0:24 VLDL, very low-density lipoproteins. 0:27 LDL, low-density lipoproteins. 0:30 And HDL, high-density lipoproteins. 0:33 Here I am just drawing how they will be represented 0:39 in this diagram. 0:42 And if we look at the composition, 0:44 or the amounts of fat and proteins 0:47 in each of these lipoproteins, we 0:49 can see that the protein content increases 0:54 from collie microns to the high-density lipoproteins. 0:59 Basically, it means that collie microns 1:02 have a lot of lipids and little proteins. 1:06 LDL, low-density lipoproteins, have a bit more proteins, 1:11 but still mainly lipids. 1:14 And then the high-density lipoproteins 1:16 have nearly half-half, half-proteins, half-lipids. 1:20 So it is actually important to understand 1:23 the different protein-to-lipid ratio 1:25 in each of these lipoproteins, because it somewhat 1:28 relates to their function. 1:30 So for example, LDL, the low-density lipoprotein, 1:34 actually contains a lot of lipids that are cholesterol. 1:38 But we say that this is bad, because if we have a lot of LDLs, 1:42 it means that we have a lot of cholesterol, which is not good. 1:48 The lipid content of HDL, so the lipid content 1:51 of the high-density lipoprotein actually 1:53 contains less cholesterol, and actually 1:56 is responsible for picking up excess cholesterol from tissue, 2:00 and then bringing it back to the liver. 2:02 And so we say HDL is good. 2:05 So LDL is bad, HDL is good. 2:09 In reality, though, LDL is not that bad. 2:12 It's just when we have high amounts of LDL that it's bad. 2:15 We need LDL. 2:17 We need cholesterol. 2:20 If this doesn't make sense, don't worry. 2:22 We will try to follow a story for each of these lipoproteins 2:25 and see what they do in our body. 2:29 Let us begin with collin-microns. 2:31 Collin-microns are responsible for transporting fats 2:34 that we have absorbed in our diet and delivering it 2:38 to body tissues. 2:40 So here we have the stomach and part of the small intestine. 2:44 Our diet consists of carbohydrates, fats, and proteins, 2:48 of course. 2:49 These get digested and then absorbed 2:52 in the duodenum of the small intestine. 2:55 Let us zoom into the small intestine here. 2:58 Here I am drawing some intestinal cells. 3:01 And under these cells, we have blood vessels. 3:05 On the surface of the small intestinal cells, 3:08 so on the apical side, we have a sodium glucose 3:11 transporter, which takes in one glucose for two sodium molecules. 3:17 Glucose is then reabsorbed into the blood. 3:22 The fats in the diet arrive in the small intestine 3:25 as lipid droplets. 3:27 The lipid droplets are digested and emulsified into my cells, 3:30 which are then absorbed into the intestinal cells 3:35 as monoglycerides and fatty acids. 3:39 And also cholesterol is absorbed. 3:42 The intestinal cells will then package the monoglycerides, 3:45 fatty acids, cholesterol, and proteins 3:49 called apoproteins to form what's known as the choline micron. 3:53 So all these things are packaged up into choline microns. 3:59 Choline microns is predominantly lipids with little proteins, 4:03 with little apoproteins. 4:05 The choline microns also contain fossil lipids 4:09 within its structure. 4:11 The choline microns is actually absorbed into the lymphatic system 4:14 before moving into the blood vessel via the subclavian vein. 4:21 The choline microns then circulate around the body, 4:24 delivering lipids, delivering triglycerides 4:28 to tissues that need it for energy. 4:31 The remaining remnants of the choline microns 4:33 will then finish and arrive at the liver. 4:36 So here we have the liver. 4:39 The choline microns will bind onto LDL receptors 4:43 and will be brought into the hepatic cells. 4:48 OK, let us leave the choline microns 4:50 and look at the other lipoproteins. 4:53 And in order to do this, we have to start 4:57 where we left off with glucose. 5:00 So here we have glucose. 5:02 The liver also receives the glucose 5:04 that we absorbed earlier. 5:05 It takes it in and through the process known as glycolysis, 5:09 it makes pyruvate. 5:11 Pyruvate can move into the mitochondria of the hepatocyte 5:14 and become acylCoA. 5:16 AcylCoA can then get transported out of the mitochondria 5:21 and can be eventually converted to cholesterol 5:24 using an important enzyme called HMGCoA reductase. 5:30 Please know that we are missing many chemical reactions, 5:32 but this is just a simple overview. 5:36 So cholesterol is made through acylCoA. 5:38 An important reaction step is with the enzyme HMGCoA reductase. 5:45 There is actually a cholesterol lowering agent 5:48 that specifically targets this enzyme called statins. 5:51 So statins are HMGCoA reductase inhibitors, 5:55 thus stopping the synthesis of cholesterol. 5:58 Going back to acylCoA, acylCoA can also 6:02 be converted to malonylCoA to form later fatty acids. 6:09 Glycolysis can also synthesize glycerol. 6:14 When glycerol and one fatty acid combine, 6:17 they form what's known as monoacylglycerol. 6:20 Then with another two fatty acids can form triacylglycerol. 6:29 I only wrote triglycerol to make it fit, 6:32 but it's actually meant to be triacylglycerol, 6:35 also known as triglyceride. 6:38 The triglyceride cholesterol, as well as 6:42 aper proteins and phospholipids, 6:44 then can get packaged up through the Golgi apparatus 6:47 to form a lipoprotein. 6:49 So lipoproteins can look something like this. 6:53 It contains proteins, the aper proteins, triglycerides, 6:58 phospholipids, and cholesterol. 7:03 The liver cannot actually make all the lipoproteins. 7:07 They actually make two. 7:09 They make either the empty HDL whose main function 7:14 is picking up excess cholesterol from the body 7:17 and bringing it back to the liver. 7:20 Or the liver can make VLDL, very low-density lipoproteins, 7:25 whose main function is for the transportation of fatty acids, 7:30 transportation of the triglycerides to body tissues. 7:34 Remember that VLDL and MTHDL have different lipid concentration 7:44 or composition. 7:45 They are completely different. 7:47 VLDL has a lot more lipids, especially triglycerides, 7:51 because VLDL will go into the blood and transport 7:56 these triglycerides to body tissues 7:58 that need it for energy or for storage. 8:02 There is adipose tissue that stores fat. 8:06 And there are tissues that use fatty acids for energy. 8:11 VLDL comes across lipases, which 8:14 will liberate the fatty acids. 8:16 And this will basically change the VLDL to become ideal. 8:21 Also known as intermediate density lipoprotein. 8:25 The fatty acids that are liberated 8:26 can either be stored as triglycerides and adipose tissue, 8:30 or used as energy by some tissues, 8:33 including the hot muscle cells. 8:37 The intermediate density lipoprotein, 8:39 the ideal can be converted through circulation 8:45 to become LDL. 8:47 LDL's main function is to transport 8:51 cholesterol to body tissues. 8:53 This means that it contains a lot more cholesterol 8:59 than any other lipoproteins. 9:02 So here are some tissues. 9:04 Tissues or cells need cholesterol 9:06 to make hormones, as well as maintaining 9:10 cell membrane integrity. 9:13 So after LDL gives these tissues cholesterol, 9:18 LDL can then return to the liver 9:20 by binding onto LDL receptors. 9:23 And it gets endostized. 9:26 So here we have our LDL. 9:28 The LDL can then either be recycled 9:31 in the Golgi apparatus to make more lipoproteins, 9:35 or can be excreted through bile. 9:37 Excess cholesterol gets excreted in bile, 9:42 because the body knows not to have 9:47 knows when there is a lot of cholesterol. 9:49 And so this is one way of basically excreting cholesterol 9:53 through bile. 9:54 Now finally, we go to our last lipoprotein, 9:58 which is the HDL. 9:59 Here we have the empty HDL. 10:01 The MTA empty HDL's main function 10:04 is to pick up excess cholesterol, or cholesterol in general. 10:09 So it enters the circulation. 10:11 And here we have tissues that have excess cholesterol, 10:14 or cells that have excess cholesterol. 10:17 And the HDL will pick up these excess cholesterol 10:20 and basically return it to the liver. 10:24 The full HDL with the cholesterol 10:27 will bind onto receptors called scavenger receptors 10:30 on the hepatocytes. 10:32 And then it will get endostized. 10:34 The HDL can then either be recycled through the Golgi, 10:41 or excreted, depending on how much cholesterol 10:43 the body needs. 10:45 So that was it. 10:46 I hope it made sense. 10:47 And I hope you understood the functions of the Coli microns, 10:50 the VLDL, the LDL, and HDL. 10:56 And also, thank you for watching. 10:59 Bye.
