3.1.2 Carbohydrates

Question 1

What type of bond holds monosaccharide molecules together in a polysaccharide?

Answer 1

Glycosidic bond

Question 2

Name the two polysaccharides present in starch.

Answer 2

Amylose and Amylopectin

Question 3

Describe the iodine test for starch.

Answer 3

• Iodine (dissolved in potassium iodide solution) is added to the test sample.
• If the starch is present, the sample changes from browny-orange to a dark, blue-black colour.

Question 4

Maltose is a sugar. Describe how a molecule of maltose is formed.
[3 marks]

Answer 4

• Two molecules of alpha-glucose…
• …are joined by a glycosidic bond.
• A molecule of water is released / a condensation reaction takes place.

Question 5

Describe the test used to identify a non-reducing sugar. Include the different results you would expect to see if the test was positive or negative.
[5 marks]

Answer 5

• Take a new sample of the test solution, add dilute HCl and heat it in a water bath that’s been brought to the boil.
• Neutralise it with sodium hydrogencarbonate.
• Add blue Benedict’s solution
• If the test is positive for a non-reducing sugar, a brick-red precipitate will form.
• If the test is negative, the solution will stay blue.

Question 6

Give the function of a carbohydrate.

Answer 6

• Carbohydrates are used to store and supply energy.

- They act as structural components of cells.

Question 7

The student produced the sucrose solutions with different concentrations from a concentrated sucrose solution.
Name the method she would have used to produce these sucrose solutions.
[1 mark]

Answer 7

Dilution series

Question 8

Describe two differences between the structure of cellulose molecule and a glycogen molecule.
[2 marks]

Answer 8

Any two from:

• Cellulose is made up of β-glucose (monomers) and glycogen is made up of α-glucose (monomers).
• Cellulose molecule has a straight chain and glycogen is branched/coiled.
• Glycogen has 1,2- and 1,6- glycosidic bonds and cellulose has only 1,4- glycosidic bonds.

Question 9

Describe and explain two features of starch that make it a good storage molecule.
[2 marks]

Answer 9

Any two from:

• Insoluble (in water), so doesn’t affect water potential.
• Branched/coiled/(α-)helix, so makes molecule compact
• Polymer of (α-)glucose so provides glucose for respiration.
• Branched for fast breakdown.
• Large (molecule), so can’t cross the cell membrane.