Lipids belong to a broad class of animal- and plant-derived compounds which include fats and oils. They are insoluble in water because they are composed primarily of long chains of hydrocarbons. Some lipids, such as fats, are solid or semisolid at room temperature while others, such as oils, are liquids. Examples of naturally derived lipids include butter, lard, tallow and fish oil from animals and cottonseed, peanut, soy, and corn oil from plants. Lipids are important dietary constituents and are present in many foods; in addition they are used in the production of many industrial products such as cosmetics, cleansers and lubricants.
Depending on their chemical composition, lipids are classified as hydrocarbons, simple lipids, or complex lipids. Hydrocarbons are the most basic form because they only contain carbon and hydrogen. Simple lipids consist of carbon, hydrogen and oxygen; complex lipids contain one or more additional elements, such as phosphorus, nitrogen, or sulfur. Examples of the latter include sterols and phospholipids.
The first scientific study of natural lipids is credited to the French chemist Michel Eugéne Chevreul. In 1811 Chevreul analyzed potassium soap made from pig fat and isolated a fatty material with acid properties. By working with soaps made from different animal fats, he found that a series of these fatty acids existed. He also found that the properties of these lipids depended on the structure of the hydrocarbon chain making up the fatty part of the molecule. By 1816 he had established that animal fats were composed not only of fatty acids but also of glycerol, an alcohol that was known to exist in olive oil and other vegetable and animal fats.
Scientists subsequently found that fatty acids can combine with glycerol to form triglycerides, one of the most common dietary fats. Such fats are classified as saturated or unsaturated depending on their chemical structure. The degree of saturation, also known as degree of hydrogenation, refers to the number of hydrogen atoms that are associated with the chemical bond between two carbon atoms. A carbon - carbon double is said to be unsaturated because the bond can react with additional hydrogen atoms. Lipids are unsaturated if they contain one or more of these double bonds. They are said to be saturated when all the double bonds have been fully hydrogenated, in other words when they have been "filled up" with hydrogen atoms.
Lipids aid in a number of biochemical functions in both plants and animals. For example, fatty lipid deposits below the skin help insulate mammals from cold temperatures. Various hydrocarbons and waxy lipids in animal skin, insect exoskeleton cuticle, and the waxy layers that coat plant leaves and fruit help prevent moisture loss. Complex lipids participate in hormonal responses in mammals, and certain fatty acid derivatives serve as sex attractants and growth regulators in insects. Furthermore, cholesterol, a complex lipid, is the basis for sex hormones of higher animals.
In humans, the liver can synthesize necessary lipids from other lipids that are ingested, or they can be made from carbohydrates and amino acids. Even with a fat free diet the body can still produce at least some of the lipids it requires to function. Still, some proportion of a healthy diet should include lipids, usually in the form of triglycerides. Essential dietary lipids include certain polyunsaturated fatty acids as well as the vitamins A, D, E, and K. However, diets high in fats are known to cause serious health problems such as arteriosclerosis, heart disease and cancer.
Industrially, lipids are processed from animal and plant sources on a large scale. They are isolated and purified by a number of techniques and may be bleached to remove undesirable color or odor. The purified lipids may then be reacted with other organic compounds to create materials with the desired functionality. Ultimately, these materials are used to make soaps, detergents, lubricants, thickeners, and a host of other raw materials which are subsequently used in cosmetics, foods, pharmaceuticals, and agricultural products.
New research on fats and oils has continued into the 1990s. A 1997 report details ttempts by genetic engineers to develop enzymes which will allow plants to convert saturated fats and oils to healthier unsaturated materials. In addition to nutritional applications, advances have been made on oils for other applications. Because they are environmentally friendly, renewable resources, an increasing number of plant oils are being evaluated for use as industrial lubricants. Two examples which are anticipated to have a significant industrial impact are the well known soybean oil and the lesser known meadowfoam oil.
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