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Carbohydrates are chemical compounds that act as the primary biological means of storing or consuming energy, other forms being fat and protein. Carbohydrates are the main source of energy for all body functions and are necessary for the assimilation of other nutrients. The primary function of carbohydrates is for short-term energy storage. A secondary function is intermediate-term energy storage. Other carbohydrates are involved as structural components in cells, such as cellulose which is found in the cell walls of plants.

Carbohydrates, as a class, are the most abundant organic compounds found in nature. They are produced by green plants and by bacteria using the process known as photosynthesis, in which carbon dioxide is taken from the air by means of solar energy to yield the carbohydrates as well as all the other chemicals needed by the organisms to survive and grow. Carbohydrates come from a wide array of foods - bread, beans, milk, popcorn, potatoes, cookies, spaghetti, corn, and cherry pie. They also come in a variety of forms. They may be in simple (sugars) or complex (starches and fibre) form. The most common and abundant are sugars, fibers, and starches.

Monosaccharides

Glucose - The most common monosaccharide is glucose. and this is the most important one for living organisms. Glucose is the main sugar metabolized by the body for energy. The body digests carbohydrates in foods, transforming them into glucose, which serves as the primary fuel for the brain and muscles. Glucose is also called blood sugar as it circulates in the blood at a concentration of 65-110 mg/mL of blood. The most common form of this sugar is called dextroglucose, commonly referred to as dextrose. Glucose is absorbed into the bloodstream through the intestinal wall. Only the mono-saccharides glucose, fructose and galactose are absorbed in humans; these are the end-products of the digestion of carbohydrates.

Galactose - Galactose (also called brain sugar) is a type of sugar found in dairy products, in sugar beets and other gums and mucilages. It is also synthesized by the body, where it forms part of glycolipids and glycoproteins in several tissues. It is considered a nutritive sweetener because it has food energy. Galactose is less sweet than glucose and not very water-soluble. Galactose is one of the hexoses in lactose. Galactose resembles glucose in chemical structure. Glucose is the main sugar metabolized by the body for energy. Galactose can easily be converted into glucose when needed for energy and can be formed from glucose.

Fructose - Commonly known as fruit sugar, fructose is a simple carbohydrate widely distributed in organism, plants, and animals. Fructose in the body may be changed into glucose by the liver and intestines. Fructose is often recommended for, and consumed by, people with diabetes mellitus or hypoglycemia, because it has a very low Glycemic Index (GI 32) relative to cane sugar (sucrose). Honey, tree fruits, berries, melons, and some root vegetables such as: beets, sweet potatoes, parsnips and onions, contain fructose; usually in combination with sucrose and glucose.

Glucosamine - Glucosamine is an amino monosaccharide found in chitin, glycoproteins and glycosaminoglycans such as hyaluronic acid and heparan sulfate. Glucosamine and other amino sugars are important constituents of many natural polysaccharides. Glucosamine is an amino derivative of the simple sugar, glucose. Glucosamine is commonly used for the relief of pain and symptoms associated with osteoarthritis and other joint disorders. Glucosamine provides the primary substrate for both collagen and proteoglycan synthesis.

Disaccharides

Lactose - Lactose is a disaccharide of milk which on hydrolysis yields glucose and galactose. Lactose is broken down by lactase to form galactose and glucose which are then absorbed by the small intestine. Lactose is formed in the mammary glands of all lactating animals and is present in their milk. Lactose is broken down in digestive system by the help of an enzyme protein called lactase. It yields the simple sugars d-glucose and d-galactose on hydrolysis. Lactose intolerance is the inability to digest significant amounts of lactose, or milk sugar, resulting from a shortage of the enzyme lactase, which is normally produced by the cells that line the small intestine.

Maltose - Maltose is a fermentable sugar produced by conversion of the starch of sprouting barley grains by malt enzymes, principally diastase. It is the fundamental structural unit of glycogen and starch and is used as a nutrient and sweetener. Maltose is the primary starch degradation product and will be further processed to alcohol and carbon dioxide during fermentation. Through a process called fermentation, glucose, maltose and other sugars are converted to ethanol by yeast cells in the absence of oxygen.

Sucrose - Sucrose is common disaccharide which functions as a transport sugar in plants. Sucrose is the common chemical name for table sugar. Sucrose is a disaccharide; each molecule of sucrose consists of two "simple sugars" (monosaccharides). Sucrose is broken down in the gut by acidic hydrolysis into its component sugars, fructose and glucose, which are then absorbed into the bloodstream through the intestine. Sucrose occurs naturally in many green plants as a product of photosynthesis.

Oligosaccharides and polysaccharides

Oligosaccharides and polysaccharides are composed of longer chains of monosaccharide units bound together by glycosidic bonds. The distinction between the two is based upon the number of monosaccharide units present in the chain. Oligosaccharides typically contain between three and nine monosaccharide units, and polysaccharides contain greater than ten monosaccharide units. Definitions of how large a carbohydrate must be to fall into each category vary however.

Polysaccharides are complex carbohydrates, made up of multiple sugar molecules. This term is commonly used only for those containing more than ten monosaccharide residues. Examples of polysaccharides include starch, dextrin, glycogen, cellulose and chitin. Glycosaminoglycan (GAG) is the polysaccharide unit that makes up proteoglycans, a molecule made of saccharides and proteins. GAGs are extracellular matrix molecules that help give tissues like cartilage their rigid structure. GAGs include chondroitin sulphate, dermatan sulphate, keratan sulphate, heparan sulphate, heparin, and hyaluronan.

Starches are polymers of glucose in which glucopyranose units are bonded by alpha-linkages. Amylose consists of a linear chain of several hundred glucose molecules. Amylopectine is a branched molecule made of several thousand of glucose units. The body must convert starch into glucose which can be utilized for immediate energy or converted to glycogen and stored in the liver for later energy needs. Potato, rice, wheat, and maize are major sources of starch in the human diet.

Glycogen is a polysaccharide made up of repeated glucose units. When carbohydrate energy is needed, glycogen is converted into glucose for use by the muscle cells. Glycogen is the chief source of stored fuel in the body. In humans and other vertebrates, most glycogen is found in the skeletal muscles, but it is found in the highest concentration in the liver. Muscle cell glycogen appears to function as an immediate reserve source of available glucose for muscle cells. Other cells that contain small amounts use it locally as well.

Cellulose is a long-chain polymer polysaccharide carbohydrate, of beta-glucose. It forms the primary structural component of plants and is not digestible by humans. Cellulose is a carbohydrate that comprises much of a plant's cell, especially the cell wall. Cellulose is a polymer made with repeated glucose units bonded together by beta-linkages. Humans and many other animals lack an enzyme to break the beta-linkages, so they do not digest cellulose. Certain animals can digest cellulose, because bacteria possesing the enzyme are present in their gut.

Oligosaccharides are corroborates that contain 3-10 saccharide units. The important oligosaccharides are raffinose and stachyose. They contain a few repeating units of glucose, fructose and galactose. Oligosaccharide can be found on cell membranes and surfaces and they function as cell markers. Oligosaccharides are considered as functional foods, since non-digestible oligosaccharides seem to be useful as prebiotics, which stimulate the growth of beneficial bacteria in the intestine. Oligosaccharides are often manufactured through enzymatic synthesis. Fructo-oligosaccharides (FOS) and inulin, which are found in many vegetables, consist of short chains of fructose molecules. Galacto-oligosaccharides (GOS), which also occur naturally, consist of short chains of galactose molecules. These compounds can be only partially digested by humans.

Sugars and carbohydrates

In biochemistry, a sugar is the simplest molecule that can be identified as a carbohydrate. These include monosaccharides and disaccharides, trisaccharides and the oligosaccharides; these being sugars composed of 1, 2, 3 or more units. In general use, "sugar" is taken to mean Sucrose, also called "table sugar" or saccharose, a disaccharide which is a white crystalline solid. Some other sugars are fructose, which is found in fruits; lactose, which is found in milk; and glucose, which is the most common sugar in the bodies of animals and plants. Table sugar or sucrose is extracted from plant sources. The most important two sugar crops are sugarcane (Saccharum spp.) and sugar beets (Beta vulgaris). Raw sugars are yellow to brown sugars made from clarified cane juice, boiled down to a crystalline solid with minimal chemical processing. Mill white sugar, also called plantation white, crystal sugar, or superior sugar, is raw sugar whose colored impurities have not been removed, but rather bleached white by exposure to sulfur dioxide. White refined sugar is the most common form of sugar in North America and Europe. Refined sugar can be made by dissolving raw sugar and purifying it with a phosphoric acid method similar to that used for blanco directo, a carbonatation process involving calcium hydroxide and carbon dioxide, or by various filtration strategies. It is then further decolorized by filtration through a bed of activated carbon or bone char depending on where the processing takes place. Brown sugars are obtained in the late stages of sugar refining (stopping the refinement before sugar becomes white and free of molasses), or by coating white refined sugar with a cane molasses syrup. Refined sugars provide calories, but lack vitamins, minerals, and fiber. Such simple sugars are often called "empty calories" and can lead to weight gain. Also, many refined foods, such as white flour, sugar, and polished rice, lack B vitamins and other important nutrients unless they are marked "enriched." It is healthiest to obtain carbohydrates, vitamins, and other nutrients in as natural a form as possible -- for example, from fruit instead of table sugar.

Simple carbohydrates and complex carbohydrates