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¡¡¡¡Reading Material 3 Lipids¡¡¡¡Introduction¡¡¡¡Lipids are a diverse group of fats and fatlike substances. The word "lipid" isdefined as "any group of substances that are soluble in organic solvents such as hy-drocarbons£¬ benzene£¬ alcohols£¬ ether£¬ and chloroform£¬ but are only sparinglysoluble in water£¬ they constitute the principal structural components of living ceilswith proteins and carbohydrates. They also include some related and derived com-pounds" . Like the other two primary organic compounds carbohydrates and pro-teins£¬ the lipids are also made up of carbon£¬ hydrogen and oxygen.Types and Structures¡¡¡¡There are many different types of lipids£¬ and each of them has different char-acteristics. They include fats and oils£¬ cholesterol£¬ phospholipids and glycolipids.¡¡¡¡Fats and oils are available in a variety of forms. Butter£¬ cooking oils marga-rines£¬ salad oils£¬ and shortenings are essentially all-lipid forms. Ingested fats andoils include not only those from obvious source but also those from invisible fat sources£¬such as cereals£¬ cheese£¬ eggs£¬ fish£¬ meat£¬ milk£¬ and nuts. Triglycerides are thechemical form in which most fat exists in food as well as in the body. It is also themain constituent of vegetable oil and animal fats. They are formed from a singlemolecule of glycerol£¬ combined with three molecules of fatty acid. When the bodyrequires fatty acids as an energy source£¬ the hormone glucagon signals the break-down of the triglycerides by hormone-sensitive lipase to release free fattyacids. Blood triglyceride levels spike right after we eat£¬ levels drop over the nextfew hours as the triglycerides are taken up and used for energy. Those which are notneeded are immediately transported to fat cells for storage. Excess triglycerides inplasma are called hypertriglyceridemia. It's linked to the occurrence of coronary ar-tery disease in some people. Elevated triglycerides may be a consequence of otherdisease£¬ such as untreated diabetes mellitus. Like cholesterol£¬ the triglyceride lev-els can be detected by plasma measurements. These measurements should be made after an overnight food and alcohol fast.¡¡¡¡Cholesterol is a white crystalline substance£¬' C27H45OH£¬ found in animal tis-sues and various foods. Cholesterol is a normally synthesized by the liver. A highlevel of cholesterol in the blood-hypercholesterolemia is a major risk factor for coro-nary heart disease£¬ which leads to heart attack. One of the major uses ofcholesterol is the synthesis of bile acids. These are synthesized in the liver fromcholesterol and are secreted in the bile. They are essential for the absorption of fatfrom the contents of the intestine. The body uses cholesterol to produce many hor-mones£¬ vitamin D£¬ and the bile acids that help to digest fat£¬ but it takes only asmall amount of cholesterol in the blood to meet these needs. If a person has toomuch cholesterol in the bloodstream£¬ the excess may be deposited in arteries£¬causing them to become thicker£¬ harder£¬ and flexible so they are unable to dotheir job-transporting blood efficiently. The result is an increased risk for heart at-tack and other forms of heart disease£¬ such as hardening of the arteries £¨athero-sclerosis£©. The amount of cholesterol in the blood can be measured by a simple£¬nearly painless blood test. This test measures your total cholesterol level as well asthe levels of cholesterol's two main components£º low-density lipoprotein £¨LDL£©and high-density lipoprotein £¨HDL£©.¡¡¡¡¡­¡­