Carbohydrates - Disaccharides and Polysaccharides - 1.3

Question 1

What is a Disaccharide?

Answer 1

A pair of monosaccharides combined.

Question 2

What does glucose joined to glucose form?

Answer 2

Maltose

Question 3

What does glucose joined to fructose form?

Answer 3

Sucrose

Question 4

what does glucose joined to galactose form?

Answer 4

Lactose

Question 5

What is a condensation reaction?

Answer 5

When monosaccharides join and a molecule of water is removed.

Question 6

What type of bond is formed from a condensation reaction of monosaccharides?

Answer 6

GLYCOSIDIC BOND

Question 7

What is hydrolysis?

Answer 7

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

Question 8

What are some examples of non-reducing sugars?

Answer 8

Some disaccharides, such as sucrose, are known as non-reducing sugars.

Question 9

Why are some disaccharides, such as sucrose, known as non-reducing sugars?

Answer 9

They do not change the colour of Benedict’s reagent when they are heated with it.

Question 10

What must be done first in order to detect a non-reducing sugar?

Answer 10

In order to detect a non-reducing sugar it must first be hydrolysed into its monosaccharide components by hydrolysis.

Question 11

What is the test for non-reducing sugars?

Answer 11

• First conduct Benedict’s test for reducing sugars, to check if a reducing sugar is present.
• Add 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 five minutes.
• Slowly add some sodium hydrogencarbonate solution to the test tube in order to neutralise the hydrochloric acid (Use pH paper to check the solution is alkaline).
• Re-test the solution by heating it with Benedict’s reagent in a gently boiling water bath for five minutes.
• If a non-reducing sugar was present in the original sample, the Benedict’s reagent will now turn orange-brown.

Question 12

What does the hydrochloric acid do in the test for non-reducing sugars?

Answer 12

The dilute hydrochloric acid will hydrolyse any disaccharides present into its constituent parts.

Question 13

What are polysaccharides?

Answer 13

Polymers formed by combining together many monosaccharide molecules.

Question 14

Are polysaccharides soluble or insoluble?

Answer 14

Insoluble

Question 15

What are some common uses for polysaccharides?

Answer 15

• Storage

- Structural support in plants

Question 16

What is starch?

Answer 16

A polysaccharide that is found in many parts of plants in the form of small granules or grains.

Question 17

How is starch formed?

Answer 17

By the joining of between 200 and 100,000 alpha-glucose molecules by glycosidic bonds in a series of condensation reactions.

Question 18

How is starch easily detected?

Answer 18

By its ability to change the colour of the iodine in potassium iodine solution from yellow to blue-black.

Question 19

In which temperature should the test for starch be carried out?

Answer 19

Room temperature

Question 20

What is the test for starch?

Answer 20

• Place 2cm3 of the sample being tested into a test tube (or add two drops of the sample into a depression on a spotting tile).
• Add two drops of iodine solution and shake or stirs.
• The presence of starch is indicated by a blurb;ack colouration.