Glycosides are a class of complex chemical compounds in plants. They are broken down by plant enzymes into sugars, among which glucose is generally included, and into other substances. The term glucoside is often used synonymously with glycoside, but in its more specific meaning it refers to glycosides that yield glucose. Each glycoside in a plant is hydrolyzed by an enzyme, usually a specific enzyme found in the same plant. The enzyme emulsin, however, causes hydrolysis of several glycosides. The enzymes and glycosides are stored in separate plant cells until the reaction products of the glycosides are needed and the enzymes are activated. Glycosides are believed to serve several purposes in the plant. Glycosides are bitter tasting, and it is believed that they help keep birds and insects from eating seeds and fruit before they are fully grown, by which time the glycosides have been converted to sweet sugars. When a plant tissue is bruised, the enzymes hydrolyze the glycosides into products, such as phenol compounds and acids that have an antiseptic action and prevent decay of the damaged tissues. Glycosides are soluble in water and are obtained from plants by water extraction. They are mostly colorless crystalline solids with a bitter taste. Simple glycosides have been synthesized in the laboratory, and several hundred glycosides have been extracted from plants and used for many purposes. Among the important glycosides are indican, used for dyeing; digitalin, used in medicine; and the saponins, foaming agents used industrially and medicinally. Glucose, a monosaccharide sugar, C6H12O6, is found in honey and the juices of many fruits; the alternate name grape sugar is derived from the presence of glucose in grapes. It is the sugar most often produced by hydrolysis of natural glycosides. Glucose is a normal constituent of the blood of animals. Glucose is a white crystalline solid, less sweet than ordinary table sugar. Solutions of glucose rotate the plane of polarization of polarized light to the right; hence the alternative name dextrose (Latin dexter, “right”). Glucose crystallizes in three different forms. The degree of rotation of polarized light is different for each form. Glucose is formed by the hydrolysis of many carbohydrates, including sucrose, maltose, cellulose, starch, and glycogen. Fermentation of glucose by yeast produces ethyl alcohol and carbon dioxide. Glucose is made industrially by the hydrolysis of starch under the influence of dilute acid or, more commonly, under that of enzymes. It is chiefly used as a sweetening agent in the food-processing industries. It is also used in tanning, in dye baths, in making tableted products, and in medicine for treating dehydration and for intravenous feeding. Glycerol or Glycerin, C3H8O3, is a colorless, odorless, sweet-tasting alcohol, with a specific gravity of 1.26. It boils at 290° C (554° F) and melts at 18° C (64.4° F). Liquid glycerol resists freezing but may crystallize at low temperatures. It forms a solution with water in any proportion, and dissolves in alcohol in all proportions, but is insoluble in ether and many other organic solvents. The term glycerol refers specifically to the compound, the formula of which is given above, whereas glycerin may also refer to glycerite (glycerol in solution) or other solutions of or preparations made from glycerol. Simple fats and oils are esters of fatty acids and glycerol crude glycerol is purified by distillation after obtaining it as a by-product after fats and oils have been treated with alkali to form the soap. Of the annual glycerol production about 40 percent comes from soap making. The rest is synthesized from the petroleum products ally alcohol, C3H6O, or propylene, C3H6, or by the catalytic action of hydrogen on sugar at high temperature and pressure. The most common use for glycerol is in the making of alkyd resins. Other important applications are in the preparation of drugs and toilet articles, including toothpastes; as a plasticizer in cellophane; and as a moistening agent in tobacco products. Because cheaper products are now available, only 5 percent of the production of industrial glycerol goes into the making of nitroglycerol explosives. Because of its affinity for water and its high viscosity, glycerol is a valuable constituent in stamp pad inks. Pumps for petroleum products are lubricated by glycerin because it resists dissolving in petroleum liquids. Because of its high viscosity and its nonpoisonous character, glycerin is a suitable lubricant for food-processing machinery.
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