1.3 Carbohydrates - disaccharides and polysaccharides

How is maltose formed?

When two glucoses join together

How is sucrose formed?

Glucose joined to fructose

How is lactose formed?

Glucose joined to galactose

What is a condensation reaction?

Joins two molecules together with the formation of a glycosidic bond and involves the elimination of a molecule of water

What is a hydrolysis reaction?

When water is added to a disaccharide under suitable conditions, it breaks the glycosidic bond releasing the constituent monosaccharides

What are non-reducing sugars?

Other disaccharides, such as sucrose because they do not change the colour of Benedict’s reagent when they are heated with it

How do you detect a non-reducing sugar by hydrolysis?

1. If the sample is not already in liquid form, it must first be ground up in water

2. Add 2cm³ of the food sample being tested to 2cm³ of Benedict’s reagent in a test tube and filter

3. Place the test tube in a gently boiling water bath for 5 minutes. If the benedict’s reagent does not change colour (the solution remains blue), then a reducing sugar is not present

4. Add another 2cm³ of the food sample to 2cm³ of dilute hydrochloric acid in a test tube and place the test tube in a gently boiling water bath for 5 minutes. The dilute hydrochloric acid will hydrolyse any disaccharide present into its constituent monosaccharides

5. Slowly add some sodium hydrogencarbonate solution to the test tube in order to neutralise the hydrochloric acid. (Benedict’s reagent will not work in acidic conditions.) Test with pH paper to check that the solution is alkaline

6. Re-test the resulting solution by heating it with 2cm³ of Benedict’s reagent in a gently boiling water bath for 5 minutes

7. If a non-reducing sugar was present in the original sample, the Benedict’s reagant will now turn orange-brown. This is due to the reducing sugars that were produced from the hydrolysis of the non-reducing sugar

What is the structure and function of a polysaccharide?

As polysaccharides are very large molecules, they are insoluble, this feature makes them suitable for storage

Some polysaccharides, such as cellulose, are not used for storage but give structural support to plant cellsHo

How do you test for starch?

The test is carried out at room temperature

1. Place 2cm³ of the sample being tested into a test tube

2. Add two drops of iodine solution and shake or stir

3. The presence of starch is indicated by a blue-black coloration