1.3 Carbohydrates - disaccharides and polysaccharides

Question 1

What are some examples of monosaccharides joining in pairs to form disaccharides?

Answer 1

• Glucose joined to glucose forms maltose.
• Glucose joined to fructose forms sucrose.
• Glucose joined to galactose forms lactose.

Question 2

What is the bond that is formed from a condensation reaction?

Answer 2

A glycosidic bond.

Question 3

What is the test for non-reducing sugars?

Answer 3

• If the sample is not already in liquid form, it must first be ground up in water.
• Add 2cm cubed of the food sample being tested to 2cm cubed of Benedict’s reagent in a test tube and filter.
• 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.
• Add another 2cm cubed of the food sample to 2cm cubed 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.
• Slowly add some sodium hydrogencarbonate solution to the test tube in order to neutralise the hydrochloric acid. Test with pH paper to check that the solution is alkaline.
• Re-test the resulting solution by heating it with 2cm cubed of Benedict’s reagent in a gently boiling water bath for 5 minutes.
• If a non-reducing sugar was present in the original sample, the Benedict’s solution will now turn orange-brown. This is due to the reducing sugars that were produced from the hydrolysis of the non-reducing sugar.

Question 4

Why are polysaccharides suitable for storage?

Answer 4

Because they are insoluble, due to them being very large molecules.

Question 5

What is the test for starch?

Answer 5

• Place 2cm cubed of the sample being tested into a test tube or on a spotting tile.
• Add two drops of iodine solution and shake or stir.
• If starch is present, orange-brown iodine turns blue-black.