1.3 disaccharides and polysaccharides

Question 1

what is a disaccharide?

Answer 1

A pair of monosaccharides joined together in a condensation reaction with glycosidic bonds.

Question 2

Name 3 examples of disaccharides.

Answer 2

Glucose + Glucose = maltose
Glucose + Fructose = sucrose
Glucose + Galactose = lactose

Question 3

What is the test for a non-reducing sugar?

Answer 3

1) add 2cm3 of the food sample (in liquid form) to 2cm3 of benedicts reagent in a tube and filter.
2) place the test tube in a water bath of boiling water for 5 minutes. if the benedict’s reagent does not change colour then a reducing sugar is not present.
3) Add 2cm3 of the food sample to 2cm3 dilute hcl and place the test tube in boiling water for five minutes.
4) slowly add sodium hydrogencarbonate solution to neutralise the hcl. test with a ph paper to make sure solution is alkaline.
5) re-test solution with benedict’s reagent in a boiling water bath for five minutes.
6) if a non-reducing sugar was present in the original sample the reagent will now turn orange-brown due to sugars produced from hydrolysis with the non-reducing sugar.

Question 4

what is a polysaccharide?

Answer 4

• polymers of many monosaccharide molecules
• monosaccharides joined by glycosidic bonds formed in condensation reactions
• large and insoluble- good for storage

Question 5

describe starch as an example of a polysaccharide

Answer 5

• found in plants as granules/grains
• formed by the joining of 200-100,000 alpha glucose molecules by glycosidic bonds in a series of condensation reactions.

Question 6

describe the test for starch

Answer 6

1) Place 2cm3 of sample into a test tube
2) add 2 drops of iodine and shake
3) if starch is present solution will turn blue-black