 In this video, I will describe the building blocks structure and functions of lipids, including the neutral fats, phospholipids, steroids, and ecosinoids. Lipids are the class of organic chemicals that are nonpolar and therefore hydrophobic. A triglyceride is a major lipid that functions as an energy storage molecule inside fat cells, which are also known as adipocytes. Adipose tissue is the fat tissue that's made of adipose sites, the fat cells, and within each adipose site is a large droplet of fat, which is triglycerides. Each triglyceride consists of a glycerol backbone that's joined to three fatty acid chains through dehydration synthesis. There are only three positions on glycerol that can bind to a fatty acid chain, so a triglyceride has three fatty acids bound to its glycerol and cannot become any larger than that. There are a wide variety of different triglycerides based on the structure of the specific fatty acid chains. Some fatty acids are shorter or longer, and another major difference between fatty acids is the bonding pattern between the carbon atoms in the fatty acid chains. There can be single bonds in a saturated fatty acid chain or double bonds forming unsaturated fatty acids. Here we can see a comparison of the structure of a saturated fatty acid to an unsaturated fatty acid. You'll notice that a saturated fatty acid has single bonds that are joining the carbon atoms within the molecule. And because each carbon is bound to each neighboring carbon with a single bond, the maximum number of hydrogen atoms are bound to the carbon atoms within a saturated fatty acid. That's what it means to be saturated. It is saturated with hydrogen atoms. In contrast, an unsaturated fatty acid has at least one double bond joining adjacent carbon atoms. And because those carbon atoms have that double bond, there are two less hydrogen atoms in a fatty acid as a result of that double bond. There may be only one double bond in an unsaturated fatty acid, as is shown in this illustration, or there are also polyunsaturated fatty acids that have multiple double bonds. The double bonds affect the melting point of the fatty acids because double bonds make the chemical structure more rigid. These fatty acids won't be able to lie next to each other and form a solid as easily. Therefore unsaturated fat tends to be liquid at room temperature or have a lower melting point. So saturated fat has a higher melting point than unsaturated fat, and typically saturated fat is solid at room temperature. Phospholipids are molecules that are based on the same glycerol backbone that we saw in the triglyceride, and there are two fatty acid chains attached to that glycerol backbone. But in the third position of glycerol, instead of having a fatty acid chain, a phospholipid has a phosphorus-containing polar head group. So phospholipids are technically amphiphilic molecules because they have a polar region and a non-polar region. The polar region is water-loving and the non-polar region is water-fearing or hydrophobic. The hydrophilic polar head will be able to form hydrogen bonds with water and therefore is able to dissolve in water, but the fatty acid chains that are non-polar are hydrophobic and are not able to mix with water. This amphiphilic character of the phospholipids makes them useful to create the barrier around our cells known as the plasma membrane. Ecosanoids are molecules that are synthesized from a 20-carbon long fatty acid chain, an enzyme known as a cyclooxygenase will convert the fatty acid into an ecosanoid. These ecosanoids are important cell signals or messages. The examples that we see in the illustration here are prostaglandins, a type of ecosanoid that's important for signaling when a tissue is damaged. Receptors on sensory nerves will detect prostaglandins and relay in the pain signal. Similarly, prostaglandins can activate cells of the immune system to help defend against infection. Medications such as aspirin and Advil are non-steroidal anti-inflammatory medications that can decrease inflammation by inhibiting the production of prostaglandins and also act as analgesics, that is, pain-relieving medications. Steroids are another class of lipids and all the steroids share a same basic chemical structure where they are synthesized from cholesterol. So cholesterol is the base steroid that other steroid chemicals can be made from. You can see that cholesterol has four ring shapes that are linked together. The majority of the structure is carbon-bound to hydrogen, so cholesterol is a lipid, a hydrophobic molecule. Similarly, the steroid hormones that are produced from cholesterol are also hydrophobic or lipid hormones. So a hormone is a chemical message that travels throughout the body in the blood. Testosterone is a male sex hormone that's produced from the male gonads, the testes, and has effects locally in the testes to stimulate the production of sperm, but also has effects throughout the body to stimulate the masculinization of the body, stimulating muscle growth and stimulating the growth of facial hair and the growth of the larynx leading to the deeper voice in men. Similarly, progesterone is a sex steroid hormone, but progesterone is a female sex hormone. Progesterone is produced by the female gonads known as the ovaries. And one of the important functions of progesterone is to maintain the inner lining of the uterus, the endometrium, in order to provide a suitable environment for the implantation of an embryo. And so progesterone will prevent menstruation and keep the uterus prepared to maintain pregnancy.