Carbohydrates

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Monosaccharides

Monomer - A small molecule made up of just one structure.

Monosaccharide - One sugar/ a single monomer of carbohydrate

Monosaccharides are easily soluble in water and form sweet solutions, e.g. glucose, fructose and galactose.

General formula: (CH2O)n, where n is a number between 3 and 7.

Glucose and fructose are hexoses so have the formula, (CH2O)6 or C6H12O6.(http://www.oup.co.uk/oxed/secondary/sciencegrids/biology/aqaa/b321_files/321_1.jpg)

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Disaccharides

A disaccharide consits of two sugars and is formed by a condensation reaction between two monosaccharides. The hydroxal (OH) group from carbon 1 of one monosaccharide and hydrogen (H) from carbon 4 of the other monosaccharide bond to make the water molecule which is lost. The bond formed is called a glycosidic bond or an alpha 1 4 glycosidic bond. A glycosidic bond can be broken by addition of water, which is known as hydrolysis.

Alpha glucose bonds to alpha glucose to form maltose. Glucose bonds to fructose to form sucrose. Glucose bonds to galactose to form lactose.(http://www.dr-sanderson.org/images/Image2.gif)

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Test for Reducing Sugars

Monosaccharides and some disaccharides (such as maltose) are reducing sugars.

Benedict's Test

Take the sample being tested in liquid form and add Benedict's reagent, heat gently for 5 minutes.

If a red colouration is seen a reducing sugar is present.

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Test for Non-reducing Sugars

Once you have tested and found it is not a reducing sugar, shown by the blue colouration remaining after carrying out the Benedict's test.

Add the sample being tested to an equal volume of dilute hydrochloric acid and boil gently for 5 minutes. This will hydrolyse and disaccharides present. Then slowly add sodium hydrocarbonate solution to neutralise the acid. Retest the solution with Benedict's reagent heating gently, if a red colouration appears a non-reducing sugar was present in the original sample.

Test for Starch

Place the sample in a test tube or put two drops into a depression on a spotting tile then add 2 drops of iodine and shake or stir. A blue-black colouration indicates the presence of starch.

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Polysaccharides

Polymer - Made up of many molecules, several repeating subunits that are similar or identical bonded together.

Polysaccharide - Made up of many sugars, several repeating monosaccharide subunits. They are created by many condensation reactions and glycosidic bonds form between molecules. They can be macromolecules, being several thousand molecules long. Polysaccharides are insoluble in water, this makes them suitable for storage.

Examples of polysaccharides are starch, cellulose and glycogen.

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