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Digestion, Mobilization, and Transport of Fats - Part I

1D: How are fats digested, mobilized, and transported inside the body in the fed and fasted states? Created by Jasmine Rana.
Video transcript
- [Voiceover] Now, of course, there are many nutrients in food, including the imaginary cheeseburger that I have just drawn here. But in this video I want to follow the journey of one nutrient. We're gonna follow the journey of fat and how the fat contained inside of this cheeseburger is digested, transported, and stored in this imaginary man that I've drawn here. So let's say it's lunchtime, he's hungry, he takes a big bite of the cheeseburger. He chews it down in his mouth and it travels down his esophagus which I've drawn in blue here, into his stomach. It's churned a little bit more, mixed with all that acid in the stomach, and then it enters the small intestine. The small intestine is just many, many loops repeating. And all of its surface area makes this an ideal place for all of the nutrients in our food, and of course fat is no exception to this, to be absorbed. And of course, anything that cannot be absorbed will continue on in the colon, which I've drawn here in blue, and eventually you will have a trip to the bathroom. But since we are interested in how fats are digested and absorbed, we want to focus our attention here to the small intestine. So if you recall, the small intestine is lined by specialized epithelial cells, which is just a fancy way to say that there are special cells that can absorb nutrients that line the inside of this tube. And so I'm drawing kind of a representative version of one of those cells. And just to kind of orient ourselves I will remind you that this side is the lumen of the small intestine, so the hole through which all the food is travelling. And of course this is our cell. Now just as for any other nutrient, the small intestine contains an array of enzymes to break down the fat molecules into smaller pieces for the cells to absorb. And as a class, these enzymes are called lipases and some of them are secreted by the pancreas, and others are found naturally along the border of these cells. And just to kind of indicate and remind ourselves that these enzymes exist, I'm gonna draw basically a Macman-like character to represent these lipases that are ready to essentially break down these fat molecules into smaller pieces. But notably, these lipase enzymes must function in an aqueous environment. Remember, most of our body functions in a water-loving environment, but this of course poses a problem because fat molecules are very hydrophobic, so remember, when we refer to a fat molecule, we think of triacylglyerides, which I'll abbreviate as TAG here. And I won't draw out the full structure, but the important things to recognize, remember, are we have a glycerol backbone, so three carbons here that I've shown in this kind of line diagram at the corners here and they're attached via oxygens to three acyl groups, which remember are carbon double bond O and then many, many carbon and hydrogens. And of course some of these might have double bonds or whatnot, but the key idea here is that because we have many carbons and hydrogens, we have a very hydrophobic molecule. And for this reason all of these molecules are not going to want to dissolve in the aqueous environment of the small intestine. In fact, they're probably gonna all essentially group together and kind of want to bind with each other instead of dissolving into the aqueous environment. And so they'll kind of form these fatty droplets that won't kind of want to be broken up very much. To solve this problem you might recall that our body secrets something called bile from the liver once food enters the small intestine, and bile is essentially a detergent for our food. So what I mean by that is, if you've ever had anything greasy on your hands, you might realize that it's hard to just get that grease off by running your hand under water. But when you add soap, it's much easier to get out. And that's because soap has both a hydrophobic as well as a very hydrophilic, or water-loving, functional groups on its molecule. And having both of these types of functional groups allows it to essentially emulsify or essentially solubilize these fatty molecules a little bit better. So what that does is it kind of breaks up these fatty fat molecules in kind of smaller pieces, increasing the surface area for which these lipase enzymes can act upon these fat molecules. Specifically, these lipase enzymes break down these triacylglyceride molecules by cleaving these molecules at these ester linkages by adding a water of molecule across each of these spawns, and so all together the end result is that we form a free glycerol backbone, and now this glycerol backbone has a hydroxyl group because it's accepted a hydrogen from water, as well as now instead of acyl groups, we form what we call a carboxylic acid group because we're adding the remaining hydroxyl group from the water to these molecules. And of course we form three of these, and we call these specific carboxylic acid groups when they come from a fat triacylglyceride, we call these fatty acids, appropriately since of course they're coming from a fat molecule. Now at this point these molecules are small enough to be able to diffuse into the intestinal cell. Now although we've officially absorbed these molecules, we aren't finished, because of course we know that we need to be able to deliver these fatty acids to the tissues of our body, most notably the adipose tissue where we can store fat for later energy use. So how do we do that? Well, the first step once we get these molecules into the small intestine is to turn them back to triacylglycerides, because you want to essentially pack them in a compact unit so that we can send them off to various tissues. So once we reform these ester linkages and reform these triacylglyceride molecules, we then want to package them, get them ready for transport. And the way that our cells do this is by using a carrier molecule called a lipoprotein. And essentially what this is is, the body essentially packages all of the triacylglycerides along with any other hydrophobic substances that the body absorbs, such as cholesterol, into the core of a protein molecule, which I'm indicating here by these purple circles. And the benefit of this is that these proteins have polar heads here. That means that they interact with the aqueous environment inside of the blood stream, for example, but inside, they're relatively hydrophobic enough to keep all of these hydrophobic molecules within them. Notably, the specific name that we call this particular lipoprotein that's produced within the intestinal cell from absorbed fats, we call this a chylomicron. So once all of these triacylglycerides are packaged into the chylomicron, the next step is for this chylomicron to leave the cell. But here's where things get a bit interesting. So let me scroll down a bit to give us some room at the bottom here. Recall that all of these cells are surrounded by capillaries, so they have a source of oxygen and nutrients, but they're also surrounded by these specialized lymphatic capillaries called lacteals, and I'll go ahead and draw one right next to it here and label that. So this here is called a lacteal, and it's a lymphatic capillary. And this right here is just our normal, regular capillary. Now notably these chylomicrons are kind of big and bulky molecules, and the capillaries have very tiny fenestrations or essentially gaps to allow molecules to be absorbed. Now these tiny gaps are small enough to allow capillaries to absorb proteins and carbohydrates, but they're way too small to absorb chylomicrons, and so the chylomicrons choose a different route. They are instead taken up by these lacteals, which have much larger pores relative to the capillary and that allows these chylomicrons to travel within them. So now that we know that these chylomicrons travel through the lymphatic vessels by entering the lacteals, you might be wondering, where do they go next? And to answer this question we just need to remind ourselves with the anatomy of the lymphatic vessels are. And the truth is we have lymphatic vessels all over our body that drain many sites of our body. Some of them in our legs, in our thorax, some of them coming from our arms, and even some draining down from our head, as well. But the key point is that most of these end up coalescing in a region that's near our neck and our shoulder, and at that point they form ducts that empty into veins. So I'm gonna just draw a vein next to each of these two points here. And most of our body drains the lymph into this side, on the left side called the left thoracic duct. But some of the lymph are also drained into this right thoracic duct here, as well. But the key point is is that eventually anything that goes into these lymphatic vessels will drain into veins in this part of your body. And so with that brief overview of fat digestion and absorption of the small intestine, and then subsequent packaging into chylomicrons and traveling through the lymphatic vessels up to the veins in our body, we'll go ahead and take a pause and pick up and continue the journey of the chylomicron in the blood vessels in our next video.