rate of fat metabolism because, when triglycerides are deposited in the liver, the rate of phospholipid formation increases. Also, certain specific chemical substances are needed for the formation of some phospholipids. For instance, choline, either obtained in the diet or synthesized in the body, is needed for the formation of lecithin, because choline is the nitrogenous base of the lecithin molecule. Also, inositol is needed for the formation of some cephalins.
Specific Uses of Phospholipids. Several functions of the phospholipids are the following: (1) Phospholipids are an important constituent of lipoproteins in the blood and are essential for the formation and function of most of these; in their absence, serious abnormalities of transport of cholesterol and other lipids can occur.
(2) Thromboplastin, which is needed to initiate the clotting process, is composed mainly of one of the cephalins.
(3) Large quantities of sphingomyelin are present in the nervous system; this substance acts as an electrical insulator in the myelin sheath around nerve fibers. (4) Phos-pholipids are donors of phosphate radicals when these radicals are needed for different chemical reactions in the tissues. (5) Perhaps the most important of all the functions of phospholipids is participation in the formation of structural elements—mainly membranes—in cells throughout the body, as discussed in the next section of this chapter in connection with a similar function for cholesterol.
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