Condensation, hydrolysis and glycosidic bondsEdexcel International A Level Biology: Subtopic test

1. 1
   Sucrose is the main sugar transported in the phloem of a sugar-cane plant. The plant makes sucrose in its leaves from two monosaccharides.

   (a)

   Which two monosaccharides join to form a molecule of sucrose?

   [1 mark]

   • AGlucose and fructose
   • BGlucose and glucose
   • CGlucose and galactose
   • DFructose and galactose

   (b)

   What type of reaction joins the two monosaccharides together?

   [1 mark]

   • AHydrolysis, in which a water molecule is added
   • BOxidation, in which hydrogen is removed
   • CCondensation, in which a water molecule is released
   • DPhosphorylation, in which a phosphate group is added

   (c)

   Describe how a molecule of sucrose is formed from its two monosaccharides.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Milk contains the disaccharide lactose. A man with lactose intolerance has too little of the enzyme lactase in his small intestine, so lactose in milk is not digested properly.

   (a)

   Which pair of monosaccharides is released when a molecule of lactose is hydrolysed?

   [1 mark]

   • AGlucose and glucose
   • BGlucose and fructose
   • CFructose and galactose
   • DGlucose and galactose

   (b)

   Which statement correctly describes the reaction catalysed by lactase?

   [1 mark]

   • AHydrolysis of a glycosidic bond, with the addition of a water molecule
   • BCondensation, with the removal of a water molecule
   • CHydrolysis of an ester bond, with the addition of a water molecule
   • DHydrolysis of a peptide bond, with the addition of a water molecule

   (c)

   Explain how lactase breaks down lactose, referring to the bond and to water.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   During germination, a barley seed releases amylase, which digests the starch stored in the seed. The embryo needs glucose for respiration. Starch is an unbranched polysaccharide in this example, made of glucose units joined by glycosidic bonds.

   (a)

   Explain how amylase and maltase together convert starch into glucose.

   [3 marks]

   (b)

   One unbranched starch chain in the seed contains 500 glucose units. Calculate the number of water molecules needed to hydrolyse this chain completely to glucose, and explain your answer.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A dairy company makes lactose-free milk by adding the enzyme lactase to milk. Lactose is a disaccharide of glucose and galactose. The company's biochemist is also studying how glycogen, a polysaccharide of glucose, is built up and broken down in liver cells.

   (a)

   Describe how lactose is formed from its monosaccharides, and explain how lactase changes the lactose in the milk.

   [6 marks]

   (b)

   Explain how glycogen is built up from glucose and broken down again, and explain why storing glucose as glycogen is better for a liver cell than storing it as free glucose.

   [6 marks]

   Total for question 4: 12 marks

End of questions