Monosaccharides and disaccharidesAQA A-Level Biology: Subtopic test

1. 1
   Carbohydrates in the human diet include the monosaccharides glucose, fructose and galactose and the disaccharides maltose, sucrose and lactose.

   (a)

   Which of these is a disaccharide?

   [1 mark]

   • AGalactose
   • BLactose
   • CFructose
   • DGlucose

   (b)

   Which two monosaccharides are joined to form sucrose?

   [1 mark]

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

   (c)

   Name the bond that joins two monosaccharides in a disaccharide and name the small molecule that is released when it forms.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A technician has a clear, colourless solution that is either sucrose or maltose. She heats a sample with Benedict's reagent in a boiling water bath and the mixture stays blue.

   (a)

   What can the technician conclude from the result?

   [1 mark]

   • AThe solution contains a non-reducing sugar, so it is sucrose
   • BThe solution contains a reducing sugar, so it is maltose
   • CThe solution contains no carbohydrate at all
   • DThe solution contains lactose

   (b)

   She then boils a fresh sample with dilute hydrochloric acid. What must she add before repeating the Benedict's test?

   [1 mark]

   • AMore dilute hydrochloric acid
   • BDistilled water
   • CSodium hydrogencarbonate
   • DSodium chloride

   (c)

   The repeated test gives a brick-red precipitate. Explain this result.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Glucose, galactose and fructose all have the molecular formula C₆H₁₂O₆. Glucose exists as two isomers, α-glucose and β-glucose. Maltose is a disaccharide made from two α-glucose molecules. Relative formula masses: maltose 342, glucose 180.

   (a)

   Explain what is meant by isomers, and describe how α-glucose differs from β-glucose.

   [3 marks]

   (b)

   Calculate the mass of glucose formed when 3.42 g of maltose is completely hydrolysed. Show your working.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student has two unlabelled, colourless solutions of equal concentration. One contains sucrose and the other contains lactose. She has Benedict's reagent, dilute hydrochloric acid, sodium hydrogencarbonate, a boiling water bath and test tubes.

   (a)

   Describe how the student could use these materials to show which solution is sucrose, and explain her results.

   [6 marks]

   (b)

   Explain how disaccharides are formed from monosaccharides, and use maltose, sucrose and lactose to show how disaccharides differ in their monosaccharides.

   [6 marks]

   Total for question 4: 12 marks

End of questions