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Biochemical Tests for Carbohydrates

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Biochemical Tests for Carbohydrates Introduction Carbohydrates take the form of sugars which act as one of three sources of energy for the human body, the other two being proteins and fats, and are also used in the construction of organs and nerve cells. As the name suggests, carbohydrates are hydrated carbon, therefore consisting of carbon, hydrogen and oxygen atoms. Glucose is simple sugar or monosaccharide, which derives from the Greek monos, single, and sacchar, sugar. All monosaccharides generally have a molecular formula which is a multiple of the empirical formula (CH2O)n, where n is the multiple. Below is the monosaccharide ?-glucose, which has the molecular formula C6H12O6, due to the six carbon atoms, twelve hydrogen atoms and six oxygen atoms found within it. �-glucose also has the molecular formula C6H12O6, as does fructose. However, these monosaccharides have differing structural formulas, meaning that they are isomas of glucose as the atoms found within them are arranged in a different manner. This is the case for many molecules, which leads to greater chemical variety, and therefore uses of substances. Monosaccharides can be grouped according to the number of carbon atoms they contain. ...read more.


An aldehyde group can be recognised by a carbon atom having a double-covalent bond with an oxygen atom, and a single covalent bond with a hydrogen atom. Reducing sugars undergo oxidation in order to reduce a surrounding solution by removing oxygen from it and transferring electrons to it. For example, a reducing sugar will reduce the Copper II ion within Benedict's solution to Copper I oxide ( Cu - -->Cu ). Reducing sugars should, in theory, be capable of both dissolving in water and reducing the substance around it. This is because the hydrogen in the aldehyde group will have a slight positive charge owing to the polarity of the double covalent bond between the carbon and oxygen atoms. This will create hydrogen bonds between the polar sugars and the partially negatively charged oxygen atoms of water molecules, allowing the sugar to dissolve. The sugar should then go on to reduce the substance around it as it oxidises to become a carboxylic acid, gaining an oxygen atom and thereby reducing the substance around it of that oxygen atom. This may only be done if the carbonyl (aldehyde) ...read more.


4 glycosidic linkage. Highlighted in red is the exposed carbonyl or aldehyde group with can be used for reduction. This free aldehyde group is essential for reduction to occur. In maltose this is simple, as it is in glucose, fructose and starch, as the carbonyl group is free. However, sucrose does not have an exposed aldehyde group, and so cannot reduce its surroundings. Below is a structural diagram, and from this linkage of glucose and fructose, it can be seen that an aldehyde group is not free. To test whether sucrose is indeed a non-reducing sugar, it was put through hydrolysis by the addition of hydrochloric acid. This split the disaccharide forming glucose and fructose, which in turn reduced the benedict's solution as they are both monosaccharides. This was, therefore, the test for a non-reducing sugar. Conclusion From the results obtained and their explanation in the structure of the saccharide, it may be concluded that all monosaccharides and most disaccharides will be both soluble in water and reducing agents. However, there are exceptions to these generalisations, the main being that starch does not dissolve in water because of its sheer size, and that sucrose is a non-reducing sugar because of the glycosidic bond between its monomers of glucose and fructose, meaning that the necessarily exposed carbonyl group is not so. ...read more.

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