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Carbohydrates are substances with the general formula Cx(H2O)y, where x and y are variable numbers; their name carbohydrates (hydrates of carbon) comes from the fact that hydrogen and oxygen are present in the same amounts as in water.

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Introduction

pCARBOHYDRATES0 Carbohydrates are substances with the general formula Cx(H2O)y, where x and y are variable numbers; their name carbohydrates (hydrates of carbon) comes from the fact that hydrogen and oxygen are present in the same amounts as in water. Carbohydrates are divided into three main classes: Monosaccharides, Di-Saccharides and Polysaccharides. Monosaccharides * Monosaccharides are single sugar units. * Their general formula is (CH2O)n (where n is between 3-9). * They are classified according to the number of carbon atoms as trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C) and heptoses (7C). Of these, pentoses and hexoses are the most common. * Monosaccharides are important as energy sources and as building blocks for larger molecules, they are suitable for this because they are chemically reactive molecules and show a wide variety of structures, including the variation in the number of carbon atoms. Aldoses and Ketoses * In Monosaccharides, all the carbon atoms except one have a hydroxyl group attached. * The remaining carbon atom is either part of an aldehyde group, in which case the Monosaccharides is called an aldose or aldo sugar, or is part of a keto group, when it is called a ketose or keto sugar. ...read more.

Middle

Open chain and ring forms Fig 1.3 shows glucose as both its 'open chain' and ring structures. The open chain form can be straight, but because of the bond angles between carbon atoms it is possible for sugars with 5 or 6 carbon atoms to bend round and form stable ring structures. In hexoses like glucose, the first carbon atom combines with the oxygen atom on carbon atom number 5 to give a six-membered ring. Note that oxygen is part of the ring and that one carbon, carbon atom number 6, sticks up out of the ring. In pentoses, the first carbon atom joins with the oxygen atom on the fourth carbon atom to give a five-membered ring. (As shown in fig 1.4) The ring structures of pentoses and hexoses are the usual forms, with only a small proportion of the molecules existing in the open chain form at any one time. The ring structure is the form used to make disaccharides and polysaccharides. Alpha and beta isomers Fig 1.3 shows that glucose can exist in two possible ring forms, known as the alpha and beta forms. ...read more.

Conclusion

Lactose * Also known as milk sugar. * Found exclusively in milk. * Is an important energy source for young mammals. * Can only be digested slowly, and so provides a slow release of energy. Sucrose * Also known as cane sugar. * It is the most abundant disaccharide in nature. * Most commonly found in plants, where is it transported in large amounts by phloem tissue. * It is a good transport sugar as it is very soluble and so can be moved efficiently in high concentrations. * It is relatively unreactive. * It can be obtained commercially as sugar cane or the sugar we buy in shops. Reducing Sugars * All monosaccharides and some disaccharides, including maltose and lactose, are reducing sugars. * This means that they can carry out a type of chemical reaction known as reduction. * Sucrose is the only common non-reducing sugar. * Two common tests for reducing sugars are Benedict's test and Fehling's test, which make use of the ability of these sugars to reduce copper from a valency of 2 to a valency of 1. ...read more.

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