CarbohydratesEdexcel A-Level Biology B: Subtopic test

1. 1
   A brewing technician is studying how barley grains are converted into fermentable sugars. Barley grains store starch. When the grains germinate, enzymes break the starch down into maltose, which is then broken down further so that yeast can ferment the sugars.

   (a)

   Which bond is broken when a molecule of maltose is hydrolysed?

   [1 mark]

   • APeptide bond
   • BGlycosidic bond
   • CEster bond
   • DPhosphodiester bond

   (b)

   Which pair of products forms when one molecule of maltose is completely hydrolysed?

   [1 mark]

   • AOne glucose and one fructose
   • BOne glucose and one galactose
   • CTwo molecules of fructose
   • DTwo molecules of α-glucose

   (c)

   Describe what happens when a molecule of maltose is hydrolysed.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A biochemistry student is drawing the structures of three monosaccharides. She draws α-glucose, β-glucose and ribose, the sugar found in RNA.

   (a)

   Which statement about ribose is correct?

   [1 mark]

   • AIt is a pentose, with five carbon atoms in each molecule
   • BIt is a hexose, with six carbon atoms in each molecule
   • CIt is a disaccharide formed from two glucose molecules
   • DIt is a polysaccharide found in RNA

   (b)

   How does the structure of β-glucose differ from that of α-glucose?

   [1 mark]

   • Aβ-glucose has five carbon atoms and α-glucose has six
   • Bβ-glucose has no hydroxyl group on carbon 1
   • CThe hydroxyl group on carbon 1 is above the ring in β-glucose and below the ring in α-glucose
   • Dβ-glucose is a disaccharide and α-glucose is a monosaccharide

   (c)

   Ribose, maltose and glycogen are all carbohydrates. Classify each as a monosaccharide, a disaccharide or a polysaccharide, and state how these three groups differ in structure.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student is investigating milk. Lactose is the main carbohydrate in milk. A dairy company treats milk with an enzyme that hydrolyses lactose, producing 'lactose-free' milk for customers who cannot digest lactose.

   (a)

   Describe how a molecule of lactose is formed from its monomers.

   [3 marks]

   (b)

   Explain how treating the milk with the enzyme changes the carbohydrate it contains.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Glucose is the monomer of several important polysaccharides. Glycogen is stored in the liver and muscles of animals, starch is stored in plant storage organs such as potato tubers, and cellulose forms the cell walls of plant cells.

   (a)

   Explain how the structures of α-glucose and β-glucose lead to starch and cellulose having different shapes and functions.

   [6 marks]

   (b)

   A student claims that glycogen and amylopectin are identical molecules. Evaluate this claim, and explain how the structure of glycogen suits it as the energy store of an active animal.

   [6 marks]

   Total for question 4: 12 marks

End of questions