[MUSIC] Hello everyone, welcome back to my Coursera class, Biochemical Principles of Energy Metabolism. You are in the middle of week five, we spent two sessions. The first one is about adipose tissue. And the other one is about biochemical structure and properties of fat molecules. So this week, this session three for week five, is about how fat molecules can be digested and absorbed into our body's circulatory system. Again, you are looking a triacylglycerol structure, triacylglycerol. Three fatty acid on top of one glycerol molecules. So what types of fat digestion can be done throughout our system? You are looking at stomach, and the gall bladder, and pancreas and intestine. So apparently, there is no major digestion process available from your oral cavity, or even stomach, there is no lipid digestion. So, major active fat digestion occurs throughout your intestinal systems. In that case, I should mention the importance of bile acid, bile acid. So this bile acid, you are looking at one example of bile acid. Glycolic acid, this is glycolic acid. So when you see this structure, bile acid, what are good examples for bile acid? Can you see any kind of biochemical hint? Basically, this is cholesterol-based. Cholesterol-based metabolic product. So from the cholesterol metabolism, liver cells like hepatocytes can synthesize bile acid. And synthesized bile acid can be stored in your gallbladder. So when fat nutrients getting into your digestive systems and when bile acids are secreted and stored and they are ready in terms of solubilizing dietary fat droplets. So when you see this structure, so bile acids are basically very hydrophobic, and some hydrophilic, right? This molecule contains largely hydrophobic area and some hydrophilic area, right? It's quite bi-functional in terms of water solubility. So fat droplets is very, very hydrophobic. Tons of fatty acid and glycerol or triacylglycerol molecules are accumulated. There is no hydration through our hydrophobic interaction, those fat molecules are accumulated, very sticky, right? So those fat droplets from the food, supposed to be degraded into smaller pieces, smaller structures. So it's kind of these bile acids act like a detergent. By utilising hydrophobic interface and hydrophilic interface, the mixture of bile acid and dietric fat droplets can finally give us like smaller, smaller fat droplets. After this bile acid action, which is very very important to facilitate lipid digestion. And then, these triacylglycerol molecules can be degraded into three pieces of free fatty acid and glycerol backbone via a lipase. So most lipases can be synthesized and supplied from the pancreas. So major lipases can be secreted from your pancreas. And this pancreas cleaved the covalent bond does allowing the release of three high energy containing fatty acids out of triacylglycerol. Okay? So, this is like an overall flow. So, dietary fat's getting into the intestines and bile acid from ultimately liver, But stored in the gall bladder. So those bile acids acts upon the dietary fats and then, making these dietary fats drop into smaller pieces, right? And further, pancreatic lipases acts upon these dietary fats. And this, a single, a stick and ball indicates free fatty acids can be released from triacylglycerols. And they're very ready for absorption, okay? And this compartment is obviously lumenal side of your intestine, I mean small intestine. Okay, intestinal epithelial cells, they're supposed to transport and uptake those free fatty acids. So how free fatty acids can be absorbed into your intestinal epithelial cells? So basically, there are many membrane transporters, fatty acid transporters available in the side of epithelial cells. So those fatty acids getting into your intestinal epithelial cells. And very interestingly, very interestingly, those free fatty acid, when they are getting into the cytoplasm of intestinal epithelial cells, and inside the cells, triacylglycerol can be reformed. And those triacylglycerols and other cholesterol, and some protein components making lipoprotein particles. It's kind of repackaging process of absorbed free fatty acid with other molecules like cholesterol or phospholipids and the lipoproteins. So as a result, in these intestinal epithelial cells, making specialized lipoprotein particles called chylomicrons. And that chylomicron is released. That chylomicron is released into your circulatory system. Mostly, those chylomicron, lipoprotein particles getting into the lacteal system. And that means, ultimately your circulatory system and can be distributed to the target of cells for energy supply. This is the how digested and observed fatty acid instead of the simple diffusing. Those three fatty acids is repackaged into triacylglycerol based lipoprotein particles. I'm going to show you lipoprotein particles. This is the way of lipid transport inside your cells. So, lipid means fats or cholesterol, or some phospholipid and some proteins. Okay the mixture of those molecules form a lipoprotein particles. Depending on the competition, there are many variants. High density lipoprotein particles, low density, very low density or chylomicrons. In particular, the lipoprotein particles packaged from the intestinal epithelial cells is called chylomicrons. Other than that, other HDL, LDL, VLDL, lipoprotein particles are involved in the lipid and cholesterol transport from your liver to peripheral tissues or from peripheral tissues back to your liver. And that lipid transport is more mediated by those HDL, LDL, VLDL, lipoprotein particles. In the context of a lipid digestion and absorption chylomicron is the major and the main lipoprotein particle. Which is the key mediator of fat transport from digestion. So there is another source for fat energy, which is about mobilization of stored fat, stored fat in adipocyte. So, we studied about how dietary fat can be digested and absorbed. But in this case, how stored fat can be used for your bioenergetic and biochemical processes. In this case, we have to think about the energy demanding conditions like starvation condition. Under starved condition or very stressed condition, adipocyte can be metabolically activated. And triacylglycerol, let's say three fatty acid linked in glycerol backbone can be degraded through the lipase action. So free fatty acid is released, and this diacylglycerol, again, free fatty acid released. And those free fatty acids out of this lipolytic biochemical reactions can be exported from a lipocyte into bloodstream. And that free fatty acid can be utilized for energy production in other peripheral target cells. And this activation of triacylglycerolysis, so called the lipolysis, can be regulated and catalyzed by different types of lipases in your adipose site via hormonal actions or neural activation. This is the other important axis of fat supply, in this case from the stored fat compared to the previous examples for dietary fat digestion and absorption.
