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Certain carbohydrates are an important storage and transport form of s (such as Glucose and Fructose ), Disaccharide s (such as Sucrose and Lactose ), Oligosaccharide s, and Polysaccharide s (such as Starch , Glycogen , and Cellulose ). STRUCTURE as a straight-chain carbohydrate ( Fischer Projection )]] ( Fischer Projection )]] Pure carbohydrates contain Ratio , giving the general Formula C''n''(H''2''O)''n''. (This applies only to monosaccharides, see below, although all carbohydrates have the more general formula C''n''(H''2''O)''m''.) However, many important "carbohydrates" deviate from this, such as Deoxyribose and Glycerol , although they are not, in the strict sense, carbohydrates. Sometimes compounds containing other elements are also counted as carbohydrates (e.g. Chitin , which contains Nitrogen ). The simplest carbohydrates are Monosaccharide s, which are small straight-chain Aldehyde s and Ketone s with many Hydroxyl groups added, usually one on each carbon except the functional group. Other carbohydrates are composed of monosaccharide units and break down under Hydrolysis . These may be classified as Disaccharide s, Oligosaccharide s, or Polysaccharide s, depending on whether they have two, several, or many monosaccharide units. MONOSACCHARIDES See Also: Monosaccharide Monosaccharides may be divided into Aldose s, which have an Aldehyde group on the first carbon atom, and Ketose s, which typically have a ketone group on the second. They may also be divided into Triose s, Tetrose s, Pentose s, Hexose s, and so forth, depending on how many carbon atoms they contain. For instance, Glucose is an Aldohexose , Fructose a Ketohexose , and Ribose an Aldopentose . Further, each carbon atom that supports a hydroxyl group (except for the first and last) is Optically Active , allowing a number of different carbohydrates with the same basic structure. For instance, Galactose is an aldohexose but has different properties from glucose because the atoms are arranged differently. ( Haworth Projection )]] The straight-chain structure described here is only one of the forms a monosaccharide may take. The aldehyde or ketone group may react with a hydroxyl group on a different carbon atom to form a Hemiacetal or Hemiketal , in which case there is an oxygen bridge between the two carbon atoms, forming a heterocyclic ring. Rings with five and six atoms are called furanose and pyranose forms and exist in equilibrium with the straight-chain form. It should be noted that the ring form has one more optically active carbon than the straight-chain form, and so has both an ''alpha'' and a ''beta'' form, which interconvert in equilibrium. However, the carbohydrate may further react with an alcohol to form an Acetal or Ketal , in which case the two forms become distinct. This is the basic type of link between the monosaccharide units of larger carbohydrates. DISACCHARIDES See Also: Disaccharide Disaccharides are composed of two monosaccharide units bound together by a Covalent Glycosidic bond. The binding between the two sugars results in the loss of a hydrogen atom (H) from one molecule and a Hydroxyl Group (OH) from the other. The most common disaccharides are Sucrose (cane or beet sugar - made from one glucose and one fructose), Lactose (milk sugar - made from one glucose and one galactose) and Maltose (made of two glucoses). The Formula of these disaccharides is C12H22O11. 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. Oligosaccharides are found as a common form of Protein Posttranslational Modification . Polysaccharides represent an important class of biological Polymer . Examples include Starch , Cellulose , Chitin and Glycogen . NUTRITION products are rich sources of complex carbohydrates]] Strictly speaking, carbohydrates are not necessary for Human Nutrition because Protein s can be converted to carbohydrates. The traditional diet of some Culture s consists of very little carbohydrate, and these people remain relatively healthy. However, carbohydrates require less water to digest than Protein s or Fat s and are the most abundant source of energy. Proteins and fat are vital building components for body Tissue and Cells , and thus it could be considered advisable not to deplete such resources by necessitating their use in energy production. Based on evidence for risk of heart disease and obesity, the . Pages 55-56. ISBN 92-4-120916-X Very low carbohydrate diets can slow down Brain and Neural function because the Nervous System especially relies on glucose. Some problems have been cited for the long term effects of a no-carbohydrate diet for some individuals. Athletes, for instance, or those that participate in high intensity activities, will have a considerable reduction in performance, due to having little or no glycogen supplies stored in muscle tissue. Additionally, Nephrotoxicity may occur, particularly in persons that are not very well hydrated. Classification and Dairy Product s should make up the bulk of carbohydrate consumption. The USDA's ''Dietary Guidelines for Americans 2005'' dispenses with the simple/complex distinction, instead recommending fiber-rich foods and whole grains. DHHS and USDA , ''Dietary Guidelines for Americans 2005'', Chapter 7 Carbohydrates The Glycemic Index and Glycemic Load systems are popular alternative classification methods which rank carbohydrates based on their effect on Blood Glucose levels. CATABOLISM There are two major Metabolic Pathway s of carbohydrate Catabolism : # Glycolysis # Citric Acid Cycle SEE ALSO REFERENCES EXTERNAL LINKS
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