1.3 - Carbohydrates - disaccharides and polysaccharides

Question 1

Test for starch

Answer 1

• place 2cm^3 of the sample in a test tube
• add 2 drops of iodine and shake/stir
• presence of starch is indicated by a blue black colour

Question 2

What is starch?

Answer 2

A polysaccharide which is found in many parts of the plants in the form of small granules or grains (e.g. chloroplasts)

Question 3

Polysaccharide properties and benefits of them:

Answer 3

Insoluble as it’s very large and this makes them suitable for storage as when hydrolysed, polysaccharides break down into disaccharides or monosaccharides - some give structural support such as cellulose

Question 4

What are polysaccharides?

Answer 4

Polymers by which combining together many monosaccharide molecules through glycosidic bonds by condensation reactions

Question 5

Process of detecting non-reducing sugars

Answer 5

• Grind up the sample if not in liquid form
• add 2cm^3 of the food sample to 2cm^3 of Benedicts reagent in a test tube and filter
• place the test tube in a gently boiling water bath for 5 minutes and if the colour doesn’t change then a reducing sugar isn’t present
• add another 2cm^3 of the food sample to 2cm^3 of dilute HCl and then place in a gently boiling water bath (hydrolyses any disaccharides into constituent monosaccharides)
• slowly add some Na2CO3 to neutralise HCl - test with pH paper
• retest solution by heating it with 2cm^3 of Benedicts in gently boiling water bath for 5 mins
  will become orange brown if present

Question 6

What are examples of non-reducing sugars and why don’t they react with Benedict’s?

Answer 6

Sucrose doesn’t react with benedicts because it must be hydrolysed into its constituent monosaccharides to be detected

Question 7

what can non-reducing sugars test be done for and how are disaccharides established apart from monosaccharides?

Answer 7

some disaccharides such as maltose are reducing sugars and you can use the benedicts test to confirm this

Question 8

Hydrolysis reaction of 2 alpha-glucose

Answer 8

Question 9

Condensation reaction of the formation of a glycosidic bond

Answer 9

Question 10

Combination pairs of disaccharides

Answer 10

glucose + glucose = maltose
glucose + fructose= sucrose
glucose + galactose = lactose

Question 11

How are disaccharides formed and under what condition can it be broken down?

Answer 11

A condensation reaction forming a glycosidic bond and under suitable conditions it can be hydrolysed through adding water into its constituent monosaccharides