Medical brain notes 44

  Lipid Metabolism Several chemical compounds in food and in the body are classified as  lipids.  They include (1)  neutral fat,  also known as  triglycerides; (2)  phospholipids;  (3)  cholesterol;  and (4) a few others of less importance. Chemically, the basic lipid moiety of the triglycerides and the phospholipids is  fatty acids,  which are simply long-chain hydrocarbon organic acids. A typical fatty acid, palmitic acid, is the following: CH 3 (CH 2 ) 14 COOH. Although cholesterol does not contain fatty acid, its sterol nucleus is synthesized from portions of fatty acid molecules, thus giving it many of the physical and chemical properties of other lipid substances.         The triglycerides are used in the body mainly to provide energy for the different metabolic processes, a function they share almost equally with the carbohydrates. However, some lipids, especially cholesterol, the phosp...

  Transport of Triglycerides and Other Lipids from the Gastrointestinal Tract by Lymph—The Chylomicrons Almost all the fats in the diet, with the principal excep-tion of a few short-chain fatty acids, are absorbed from the intestines into the intes-tinal lymph. During digestion, most triglycerides are split into monoglycerides and fatty acids. Then, while passing through the intestinal epithelial cells, the mono-glycerides and fatty acids are resynthesized into new molecules of triglycerides that enter the lymph as minute, dispersed droplets called  chylomicrons,  whose diame-ters are between 0.08 and 0.6 micron. A small amount of apoprotein B is adsorbed to the outer surfaces of the chylomicrons. This leaves the remainder of the protein molecules projecting into the surrounding water and thereby increases the suspen-sion stability of the chylomicrons in the lymph fluid and prevents their adherence to the lymphatic vessel walls.         ...

  Fat Deposits Adipose Tissue Large quantities of fat are stored in two major tissues of the body, the  adipose tissue  and the  liver.  The adipose tissue is usually called  fat deposits,  or simply tissue fat. The major function of adipose tissue is storage of triglycerides until they are needed to provide energy elsewhere in the body. A subsidiary function is to provide heat insulation for the body. Fat Cells (Adipocytes).  The fat cells (adipocytes) ofadipose tissue are modified fibroblasts that store almost pure triglycerides in quantities as great as 80 to 95 per cent of the entire cell volume. Triglycerides inside the fat cells are generally in a liquid form. When the tissues are exposed to prolonged cold, the fatty acid chains of the cell triglycerides, over a period of weeks, become either shorter or more unsaturated to decrease their melting point, thereby always allowing the fat to remain in a liquid state. This is particularly importan...

  Use of Triglycerides for Energy: Formation of Adenosine Triphosphate About 40 per cent of the calories in a typical American diet are derived from fats, which is almost equal to the calories derived from carbohydrates. Therefore, the use of fats by the body for energy is as important as the use of carbohydrates is. In addition, many of the carbohy-drates ingested with each meal are converted into triglycerides, then stored, and used later in the form of fatty acids released from the triglycerides for energy. Hydrolysis of Triglycerides.  The first stage in using triglyc-erides for energy is their hydrolysis into fatty acids and glycerol. Then, both the fatty acids and the glycerol are transported in the blood to the active tissues, where they will be oxidized to give energy. Almost all cells—with some exceptions, such as brain tissue and red blood cells—can use fatty acids for energy. Glycerol, on entering the active tissue, is immediately changed by intracellular enzymes in...

  Formation of Acetoacetic Acid in the Liver and Its Transport in the Blood A large share of the initial degradation of fatty acids occurs in the liver, especially when excessive amounts of lipids are being used for energy. However, the liver uses only a small proportion of the fatty acids for its own intrinsic metabolic processes. Instead, when the fatty acid chains have been split into acetyl-CoA, two mole-cules of acetyl-CoA condense to form one molecule of acetoacetic acid, which is then transported in the blood to the other cells throughout the body, where it is used for energy. The chemical processes are the following: Part of the acetoacetic acid is also converted into  β -hydroxybutyric acid,  and minute quantities are converted into  acetone  in accord with the following reactions:         The acetoacetic acid,  β -hydroxybutyric acid, and acetone diffuse freely through the liver cell membranes and are transported...

  Synthesis of Triglycerides from Carbohydrates Whenever a greater quantity of carbohydrates enters the body than can be used immediately for energy or can be stored in the form of glycogen, the excess is rapidly converted into triglycerides and stored in this form in the adipose tissue. In human beings, most triglyceride synthesis occurs in the liver, but minute quantities are also synthesized in the adipose tissue itself. The triglycerides formed in the liver are transported mainly in very low density lipopro-teins to the adipose tissue, where they are stored. Conversion  of  Acetyl-CoA  into  Fatty  Acids.   Thefirst  step in  the  synthesis  of triglycerides  is  conversion  of carbohydrates into acetyl-CoA. This occurs during the normal degradation of glucose by the glycolytic system. Because fatty acids are actually large polymers of acetic acid, it is easy to understand how acetyl-CoA can be converted into...

  Synthesis of Triglycerides from Proteins Many amino acids can be converted into acetyl-CoA. The acetyl-CoA can then be synthesized into triglycerides. Therefore, when people have more proteins in their diets than their tissues can use as proteins, a large share of the excess is stored as fat.

  Regulation of Energy Release from Triglycerides Carbohydrates Are Preferred over Fats for Energy When Excess  Carbohydrates Are Available.  When excess quantities ofcarbohydrates are available in the body, carbohydrates are used preferentially over triglycerides for energy. There are several reasons for this “fat-sparing” effect of carbohydrates. One of the most important is the fol-lowing:The fats in adipose tissue cells are present in two forms: stored triglycerides and small quantities of free fatty acids. They are in constant equilibrium with each other. When excess quantities of a- glycerophosphate  are present (which occurs when excess carbohydrates are available), the excess a-glycerophosphate binds the free fatty acids in the form of stored triglycerides. As a result, the equilibrium between free fatty acids and triglycerides shifts toward the stored triglycerides; consequently, only minute quantities of fatty acids are available to be used for energy. Beca...

  Phospholipids The major types of body phospholipids are  lecithins,cephalins,  and  sphingomyelin;  their typical chemical  formulas are shown in Figure 68–4.   Phospholipids always contain one or more fatty acid molecules and one phosphoric acid radical, and they usually contain a nitrogenous base. Although the chemical structures of phospholipids are somewhat variant, their physical properties are similar because they are all lipid soluble, transported in lipoproteins, and used throughout the body for various structural purposes, such as in cell membranes and intracellular membranes. Formation of Phospholipids.  Phospholipids are synthesizedin essentially all cells of the body, although certain cells have a special ability to form great quantities of them. Probably 90 per cent are formed in the liver cells; sub-stantial quantities are also formed by the intestinal epithelial cells during lipid absorption from the gut.  The rate of phospho...