Which molecule is glycerol figure 5.8

The unique structure of phospholipids makes them both fat- and water-soluble, or amphiphilic. The fatty-acids are hydrophobic dislike water , and the phosphate group and glycerol are hydrophilic attracted to water. The amphiphilic nature of phospholipids makes them very useful for several functions in the body.

Every cell in the body is encased in a membrane composed primarily of a double layer of phospholipids also known as the phospholipid bilayer , which protects the inside of the cell from the outside environment while at the same time allowing for transport of fat and water through the membrane.

Another important role of phospholipids is to act as emulsifiers. Emulsions are mixtures of two liquids that do not normally mix oil and water, for example. Without an emulsifier, the oil and water separate out into two layers.

Because of their ability to mix with both water and fat, phospholipids are ideal emulsifiers that can keep oil and water mixed, dispersing tiny oil droplets throughout the water. Lecithin—a phospholipid found in egg yolk, soybean, and wheat germ—is often used as a food emulsifier. Emulsifiers also play an important role in making food appetizing; their inclusion in foods like sauces and creams makes for a smoother texture and prevents the oil and water ingredients from separating out.

They also can extend shelf life. In this video, chef Sixto Alonso demonstrates how using an emulsifier—mustard, in this case—can allow oil and vinegar to mix and stay in solution to make a salad dressing.

Sterols have a very different structure from triglycerides and phospholipids. Most sterols do not contain any fatty acids but rather are multi-ring structures, similar to chicken wire. They are complex molecules that contain interlinking rings of carbon atoms, with side chains of carbon, hydrogen, and oxygen attached.

Cholesterol is the best-known sterol because of its role in heart disease. It forms a large part of the fatty plaques that narrow arteries and obstruct blood flow in atherosclerosis. However, cholesterol also has many essential functions in the body. Like phospholipids, cholesterol is present in all body cells as it is an important substance in cell membrane structure. Cholesterol is also used in the body as a precursor in the synthesis of a number of important substances, including vitamin D, bile, and sex hormones such as progesterone, testosterone, and estrogens.

Cholesterol is not an essential nutrient; it does not need to be consumed in the diet, because it is manufactured in the liver. Only foods that come from animal sources contain cholesterol. Cholesterol is found in foods like meat, poultry, fish, egg yolks, butter, and dairy products made from whole milk.

Plant foods do not contain cholesterol, but sterols found in plants resemble cholesterol in structure. Plant sterols occur naturally in vegetable oils, nuts, seeds, and whole grains. In addition, some foods like margarines and dressings are fortified with plant sterols. Triglycerides Triglycerides are the main form of lipids in the body and in foods. Copyright; author via source The structure of a triglyceride is made up of glycerol and three fatty acids.

Copyright; author via source Figure 5. The structure of a triglyceride is often depicted as a simplified drawing of the glycerol backbone and three fatty acids.

Phospholipids Phospholipids are found in both plants and animals but make up only about 2 percent of dietary lipids. Glycerol can enter the glycolytic cycle via conversion to dihydroxyacetone phosphate a two-step conversion using glycerol kinase and glycerolphosphate dehydrogenase.

The fatty acids are secreted from the adipose cells into the bloodstream where they bind to a carrier protein, albumin. This complex can then be brought inside of other cells by endocytosis, where they can be broken down as an energy source. The first step is performed by one of a family of enzymes known as acyl-CoA synthetases or thiokinases, and requires Coenzyme A and ATP hydrolysis. These reactions occur either on the cy- toplasmic surface of the mitochondrial outer membrane or the endoplasmic reticulum, where acyl-CoA synthetases are embedded.

In the second reaction, carnitine palmitoyl- transferase I on the outside of the inner mitochondrial membrane links the acyl chain to carnitine, releasing CoA. The acyl-carnitine is transported into the mitochondrial matrix where carnitine palmitoyltransferase II releases the fatty acyl chain from the carnitine and reattaches it to an molecule of CoA.

Carnitine deficiency syndromes can occur when there is either a dysfunctional mutation of carnitine palmitoyltransferase or a severe deficiency of intracellular carnitine. Since most of the carnitine in the body is found in cardiac and voluntary muscle, the usual symptoms are muscle weakness and cardiac arrhythmias, as well as hypoketosis.

In neonates, the arrythmias can lead to death. Carnitine supplementation is a successful treatment in systemic carnitine deficiency due to either low carnitine intake or defects in the carnitine transporter embedded in the cell membranes. However, if the defect is in the palmitoyltransferase, supplementation will be unsuccessful.

Carnitine is widely sold as a dietary supplement for increasing weight loss by enhancing fat catabolism. However, that only holds true if carnitine levels are below saturation levels for the palmitoyltransferases.

Currently, the biomedical community has not reached a consensus on the efficacy of carnitine supplementation on fatty acid oxidation in carnitine-sufficient persons. All even-numbered, fully saturated, fatty acids can thus be completely oxidized. The presence of double bonds in un- saturated fatty acids introduces complications to this process that must be addressed using additional enzymes that either move the double bond or remove it.