Hello, everyone. Welcome back to my Coursera class. This is the second session of week five, which is the fat metabolism. In the previous session, we studied about the basic structure and function of adipose tissue. For today, I'm going to give you the general overview of fat molecules. So, let's talk about the lipid. Fat is one of different types of lipid molecules available in living cells. So, lipids comprise a group of naturally-occurring macromolecules. There are many types available, fats, or some wax, or sterols, or fat-soluble vitamins A, D, E, K, or sometimes, like phospholipid and other lipid molecules found in nature. So in general, what's the main function of lipids? So depending on the type of lipids, we can produce energy. We can generate a lot of ATP molecules out of the oxidative processes, catabolic processes. In this case, fats are major lipids specialized for energy production. Some classes like vitamins is very, very important for providing critical cofactors in some target enzymes, and in the case of phospholipids, those phospholipids are main structural components of membranes, or the cell. Plasma membrane, or mitochondria, nucleus, and those membrane-based structures are made possible by the synthesis and supply of phospholipids. And lastly, cholesterol-derived steroid hormones, or sterol-based metabolites, can be used to drive, and trigger, and regulate, specific cellular signaling networks to coordinate whole-body homeostasis. So overall, the lipids are very, very unique in terms of their structures. The key feature of biochemical structure of lipid is those compounds composed of predominantly hydrogen or carbon, only hydrogen or carbon, mainly. So, because of these nonpolar elements and their structures, they exhibit hydrophobicity, that means those molecules are really, really insoluble in water. So, because of this hydrophobicity of lipid molecules, they can form their own like high molecular interactions throughout hydrophobic molecular interactions. So this case, we're looking at fatty acid, or this is general structure of cholesterol, except on a few part, mostly hydrocarbon tails, and chains, and the rings structures, can be formed from lipid molecules. In the context of bioenergetics, which is the chi topic and main topic of my course, the fatty acids should be clearly introduced. So, fatty acid for main lipids for energy production can be further classified into two types, saturated fatty acid and unsaturated fatty acid. So basically, fatty acid means such a low hydrocarbon, CH2, CH2 chains plus as its cis-trans, COO_minus carboxylic acid, carboxylic group, carboxylic acid. So, fatty acid because of the COO_minus carboxylic group. So, saturated means all hydrocarbons are single bond, C-C single bond-based. In this case, when the saturated fatty acid exist in lipid droplets whatever, and that molecules become very, very solid at room temperature because of the huge, highly, packaged hydrophobic interaction. So, what about the other class, unsaturated fatty acid? So, we call fatty acid unsaturated fatty acid depending on the presence of at least one C-C double bond. So, these unsaturated fatty acid becomes quite liquid at room temperature, very oily. We are looking at, I believe this is olive oil, plant oil. They contain quite an amount of unsaturated fatty acid. Because of the double bond, these fatty acid, sort of a dimensional structure, is quite bent. In particular, naturally-occurring unsaturated fatty acid, the configuration in cis, cis means the hydrocarbon tails, those tails around the C-C double bond This is the top of bond. They're localized on the same plane. This cis configuration indicates that unsaturated fatty acid is naturally occurring form. On the other hand, trans fatty acid, as you can see around the C-C double bond, hydrocarbon tails on the opposite side. This chain, and this chain around the C-C double bond, they're on different sides. These trans fatty acids indeed synthesized from very biosynthetic chemical processes. Because of that, those trans fats have many adverse side effects in your body, because we don't have appropriate enzymes can degrade or excrete those trans fatty acids. So, those trans fats inside your system, body system, can raise the bad LDL cholesterols, and elevate the risk of developing heart disease, and stroke, moreover the type two diabetes, trans fatty acids. So, you should avoid consuming trans fatty acids from food. So, fats. So basically, those fatty acids can be linked on top of glycerol backbone. This is glycerol, C3 compound short hydrocarbon chain. And in this diagram, you're looking at three types of fatty acids can be conjugated into a single glycerol molecules, and this is so-called tryglycerides or triacyglycerols. Simply speaking, we just call it fat. So, fats are very, very important for energy storage purposes, or for brown fats, these fats can be used to drive the heat through the thermogenesis. So, one gram of fats can store much more energy than just simply one gram of glycogen or starch. Glycogen, starch, is carbohydrate polymer. Fat is the fatty acid based lipid molecules. Why is that?Because the triacyglycerols are basically very, very hydrophobic. So, compared to glycogen or starch, which is heavily hydrated, so fat molecules can be highly concentrated. So, energy then means energy can be highly concentrated in a form of fat. So again, you're looking at the differentiated function adipocytes. This red staining indicates the accumulated tryacyglycerols, I mean their molecules. And primary function for fat is obviously to keep give us enough energy. And other than that, fats can be also quite structurally important in providing physical cushioning or temperature insulating purposes.
