1.3 Carbohydrates - disaccharides and polysaccharides

Question 1

What following pairs of sugars form maltose, sucrose and lactose?

Answer 1

maltose : glucose + glucose
sucrose : glucose + fructose
lactose : glucose + galactose

Question 2

What is a condensation reaction

Answer 2

chemical process in which 2 molecules combine to form a more complex on with the elimination of a simple substance usually water

Question 3

What type of bond is formed during a condensation reaction between monosaccharides?

Answer 3

A glycosidic bond

Question 4

What is hydrolysis?

Answer 4

the breaking down of large molecules into smaller ones by the addition of water molecules

Question 5

How do you test for non-reducing sugars?

Answer 5

Benedicts test
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 Benedictus reagent in a test tube and filter
3. place the test tube in a gently boiling water bath for 5 mins. If the Benedictus reagent does not change colour, then a reducing sugar isn’t 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 mins. The dilute HCL will hydrolyse any disaccharide present into its constituent monosaccharides
5. slowly ad some sodium hydrogen carbonate solution to the test tube in order to neutralise the HCL. (Benedictus doesn’t 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 Benedictus reagent in a gently boiling water bath for 5 mins
7. if a non-reducing sugar was resent in the OG sample, the Benedictus reagent will now turn orange-brown. Due to the reducing sugars that were produced from the hydrolysis of the non-reducing sugar.

Question 6

Why are polysaccharides insoluble?

Answer 6

Because they are very large molecules

Question 7

How do you test for starch?

Answer 7

Iodine test
1. place 2cm3 of the sample being tested into a test tube (or add 2 drops of the sample into a depression on a spotting tile)

2. add 2 drops of iodine solution and shake or stir
3. the presence of starch is indicated by a blue-black coloration