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Glycosides, 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 (converted in a reaction with water) 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

Glucose, monosaccharide sugar,  C 6 H 12 O 6 . It 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 (see Sugar Metabolism ). 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, u

The chemical reaction whereby plants such as yeast use sugar is remarkably similar to the metabolism of sugar in the human body. Yeast contains a mixture of 12 enzymes, which are collectively known as zymase. Most of these enzymes, including hexokinase, are identical to enzymes involved in the human metabolism of glucose. The principal difference occurs at the end of the chain of reactions; a glucose-decomposition product called pyruvic acid is converted in the body into lactic acid, but in plants it is converted by zymase into ethyl alcohol. See Fermentation . Many problems in the physiology of sugar remain to be solved. Present work in this field has been accelerated since the discovery of tracer elements, especially radioactive carbon. Sugars, synthesized with radioactive carbon, can be followed through the body after ingestion. More Articles: DIGESTION, ASSIMILATION, AND STORAGE ENZYMES AND HORMONES   GLYCEMIA AND GLYCOSURIA 

If the body produces too much pituitary hormone or too little insulin, the amount of sugar in the blood rises abnormally, producing a condition known as hyperglycemia. In hyperglycemia the blood may contain as much as four times the normal amount of sugar. Hyperglycemia in itself is not lethal, but it is a symptom of a serious disease, diabetes mellitus. Diabetes is sometimes caused by a tumor or other condition in the pancreas that prevents the formation of insulin. Diabetic patients do not die of hyperglycemia, but if they are not given injections of insulin they may die from such causes as the accumulation of poisons in the body, produced by altered metabolism of fats; the body of the diabetic consumes fats as a substitute for the sugar that it cannot use. If an excessive amount of insulin is injected into the body, the amount of sugar is reduced to a dangerously low level, a condition known as hypoglycemia or insulin shock. Controlled insulin shock is sometimes used in the tre

The interconversion between glucose and glycogen is catalyzed by a number of different enzymes. Phosphorylase is responsible for the release of glucose-1-phosphate from glycogen; the reaction is enhanced by the hormones adrenaline and glucagon. Glucose-1-phosphate is converted to glucose-6-phosphate, which can either be metabolized or converted to free glucose, which enters the bloodstream. The uptake of glucose by cells is stimulated by insulin . Before glucose is used it is converted to glucose-6-phosphate (by hexokinase), which may be metabolized or (in the liver and in muscle) converted to uridine diphosphate glucose. From the latter compound glucose is transferred to glycogen, in a reaction catalyzed by glycogen synthetase and stimulated by insulin. By as yet unknown mechanisms, cortical and pituitary hormones as well as thyroxin are also involved in the control of carbohydrate metabolism. More Articles: DIGESTION, ASSIMILATION, AND STORAGE GLYCEMIA AND GLYCOSURIA  FERMENTA

Carbohydrates such as starch, dextrin, glycogen (animal starch), sucrose (cane sugar), maltose (malt sugar), and lactose are broken down in the digestive tract into simple, six-carbon sugars that pass easily through the intestinal wall. Fructose (fruit sugar) and glucose are unchanged in the digestive tract and are absorbed as such. Cellulose, a common constituent of many foods, is an important nutritional element for some animals, notably cattle and termites, but has no value in human nutrition (see Nutrition, Human ). The digestion of carbohydrates is performed by various enzymes (see Enzyme ). Amylase, found in saliva and in the intestine, breaks starch, dextrin, and glycogen into maltose, a 12-carbon sugar. Other sugar-converting enzymes in the small intestine break 12-carbon sugars into 6-carbon sugars. Maltase breaks maltose into glucose; sucrase, or invertase, breaks cane sugar into glucose and fructose; lactase breaks milk sugar into glucose and galactose. The six-carb

Sugar Metabolism, process by which the body uses sugar for energy. Carbohydrates, one of the three principal constituents of food, form the bulk of the average human diet. The end product of the digestion and assimilation of all forms of carbohydrate is a simple sugar, glucose , commonly called grape sugar when found in food, or blood sugar when found in the human body. The metabolism of fats and of certain protein substances also sometimes leads to the production of glucose. Glucose is the principal fuel that the muscles and other portions of the body consume to produce energy. It is present in every cell and almost every fluid of the body, and its concentration and distribution are among the most important processes in human physiology. A few other sugars are of comparatively minor importance in human physiology, notably lactose , or milk sugar, which is formed in the mammary glands of all lactating animals and is present in their milk. See Metabolism ; Sugar. More Articles: D