Carbohydrates - disaccharides and polysaccharides

Question 1

List 3 examples of disaccharides that are formed from monosaccharides.

Answer 1

Glucose + Glucose = Maltose
Glucose + Fructose = Sucrose
Glucose + Galactose = Lactose

Question 2

What happens when monosaccharides join?

Answer 2

A molecule of water is removed through condensation reaction and glycosidic bond is formed.

Question 3

How can disaccharides break down into monosaccharides?

Answer 3

Through the addition of water, hydrolysis reaction which breaks down the glycosidic bond.

Question 4

Draw the formation and breaking of the glycosidic bond.

Answer 4

Draw on a piece of paper.

Question 5

Define non-reducing sugar.

Answer 5

Non-reducing sugars are sugars which do not have an aldehyde functional group e.g. sucrose.

Question 6

What is the difference between reducing and non-reducing sugar?

Answer 6

Reducing sugar gives off a orange-brown colour when mixed with a Benedict reagent but non-reducing sugar does not change colour.

Question 7

How to detect a non-reducing sugar?

Answer 7

The non-reducing sugar must first be hydrolysed into its monosaccharide components by hydrolysis.

Question 8

Describe the process of detecting a non-reducing sugar.

Answer 8

1) Make sure the sample is in liquid form.
2) Add 2cm3 of the food sample with 2cm3 of Benedict’s reagent in a test tube.
3) Place the test tube in a water bath and gently boil it, if it stays blue, reducing sugar is not present.
4) Add another 2cm3 food sample into 2cm3 dilute HCL acid and boil. The HCL will hydrolyse any disaccharides into monosaccharides.
5) Add some sodium hydrogencarbonate solution to test tube to neutralise the HCL acid. (Benedict’s reagent doesn’t work in acidic conditions.)
6) Re-test the solution by adding 2cm3 of Benedict’s reagent and boil.
7) If non-reducing sugar was present in the original sample, the reagent will now turn orange-brown, due to the reducing sugar which was produced from the hydrolysis of the non-reducing sugar.

Question 9

How are polysaccharides formed?

Answer 9

They are polymers, formed by combining many monosaccharides, joint by glycosidic bonds formed by a condensation reaction.

Question 10

Describe the features of polysaccharides.

Answer 10

Large molecules
Insoluble
Those features allow them to be suitable for storage.
But some polysaccharides e.g. cellulose are not used for storage but give structural support to plant cells.