carbohydates - monosaccharides,disaccharides,polysaccharides, tests.

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Carbohyrdrates are Carbon molecules (carbo) combined with water (hydrate)

In carbohydrates, a single monomer is called a monosaccharide, two monosaccharides make a disaccharide ( di means two) Lots of monosaccharides can combine to form a polysaccharide.

Monosaccharides are sweet-tasting, soluble substances that have the general formula (CH2O)n  ( the 2 and n are small and lower)  n can be any number from 3 to 7.

The best known monosaccharide is glucose.This molecule is a hexose (6-carbon) sugar and has the formula C6H1206 (all numbers are lower and smaller)

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2 monosaccharides join to form a disaccharide. for example:

glucose linked to glucose forms maltose

glucose linked to fructose forms sucrose

glucose linked to galactose forms lactose.

when two monosaccharides join, a molecule of water is removed and the reaction is called a condensation reaction.The bond that is formed is called a glycosidic bond.

when water is added to a disaccharide, it breaks the glycosidic bond, releasing the monosaccharides that the disaccharide was formed from.

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Polysaccharides are polymers,formed by combining many monosaccharide molecules. The monosaccharides are joined by glycosidic bonds that were formed by condensation reactions.

Polysaccharides are very large, which means they are insoluble. This makes them suitable for storage.

Starch is a polysaccharide which is formed by the linking of 200 to 100 000 alpha glucose molecules.

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Benedicts test


reducing sugars (all monosaccharides and some disaccharides) You add benedicts reagent to a sample and heat it. If the sample contains reducing sugars it gradually turns brick red due to the formation of a red precipitate.

non-reducing sugars:Firstly, you have to breaks the sugars into monosaccharides. You do this by boiling the test tube with HCl, then neutralise it with sodium hyrogencarbonate. Then you carry out benedict's tests as you would for a reducing sugar.

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