Monosaccharides

Monomers are small basic molecular units that can be combined to form long chains called polymers. The type of reaction which link molecules together are called condensation reactions, where a water molecule is released. The type of reaction which breaks down polymers into many monomers are called hydrolysis reactions, where water molecules are used to break the bonds between monomers.

The monomers of carbohydrates are called monosaccharides, and the polymers of carbohydrates are called polysaccharides. The bonds between monosaccharides in polysaccharides are called glycosidic bonds.

Examples of monosaccharides include fructose, galactose, and alpha and beta glucose. The difference between alpha and beta glucose is the position of the hydroxyl (OH) group. The structure of alpha and beta glucose is shown below. 

glucose, beginner biology

Disaccharides are formed through a condensation reaction between two monosaccharides, releasing one molecule of water.

There are three disaccharides. These are:

1) alpha glucose + alpha glucose = maltose

2) alpha glucose + fructose = sucrose 

3) alpha glucose + galactose = lactose

The Benedict’s test can be used to check for the presence of reducing sugars and non-reducing sugars. Reducing sugars include all monosaccharides, as well as maltose and lactose. Non-reducing sugars include sucrose and most other polysaccharides. The steps to test for reducing sugars are:

1) Heat your sample until it boils and then add Benedict’s solution. 

2) If the solution remains blue there are no reducing sugars, but if red-orange precipitate forms reducing sugars are present.

3) The darker the precipitate, the more reducing sugars are present which can be measured using a colourimeter.

The steps to test for non-reducing sugars are:

1) Heat your sample with HCl acid until the solution boils, and then neutralise the solution with sodium hydrogen-carbonate. After that, heat the sample with Benedict solution until it boils again.

2) If the solution remains blue there are no non-reducing sugars, but if a red-orange precipitate forms non-reducing sugars are present.

3) The darker the precipitate, the more non-reducing sugars are present which can be measured using a colourimeter.

 

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