LipidsEdexcel International A Level Biology: Subtopic test

1. 1
   A food scientist compares butter with olive oil. Butter is solid at room temperature and olive oil is liquid. Both are made mostly of triglycerides.

   (a)

   Which of the following molecules join together to form a triglyceride?

   [1 mark]

   • AThree glycerol molecules and one fatty acid
   • BOne glycerol molecule and three amino acids
   • COne glycerol molecule and three fatty acids
   • DThree glucose molecules and one glycerol molecule

   (b)

   Which type of bond joins a fatty acid to glycerol in a triglyceride?

   [1 mark]

   • AEster bond
   • BGlycosidic bond
   • CPeptide bond
   • DHydrogen bond

   (c)

   Olive oil contains a high proportion of unsaturated fatty acids. Explain why olive oil is liquid at room temperature whereas butter is solid.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Stearic acid and oleic acid are two fatty acids found in animal fats. Both have 18 carbon atoms in each molecule. Stearic acid has no carbon–carbon double bonds. Oleic acid has one carbon–carbon double bond.

   (a)

   Which statement correctly describes a saturated fatty acid?

   [1 mark]

   • AIts hydrocarbon chain contains one C=C double bond
   • BIts hydrocarbon chain contains two or more C=C double bonds
   • CIt has no hydrocarbon chain
   • DIts hydrocarbon chain contains no C=C double bonds

   (b)

   Which statement about oleic acid is correct?

   [1 mark]

   • AIt is saturated because it has 18 carbon atoms
   • BIt is unsaturated because it has one C=C double bond
   • CIt is unsaturated because it contains a carboxyl group
   • DIt is polyunsaturated because its chain is long

   (c)

   The formula of stearic acid is C₁₈H₃₆O₂ and the formula of glycerol is C₃H₈O₃. Deduce the molecular formula of the triglyceride formed from one glycerol molecule and three stearic acid molecules. Show your working.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Cheese made from cow's milk is rich in triglycerides. These are digested in the small intestine by the enzyme lipase. Stearic acid, C₁₈H₃₆O₂, is one of the fatty acids in the triglycerides and has no C=C double bonds.

   (a)

   Oleic acid, another fatty acid in cheese, also has 18 carbon atoms but has one C=C double bond. Deduce the number of hydrogen atoms in a molecule of oleic acid and explain your answer.

   [3 marks]

   (b)

   Describe what happens to a triglyceride when lipase digests it, and state the number of water molecules needed for each triglyceride molecule.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A nutritionist compares coconut oil, which is solid at 20 °C and rich in saturated fatty acids, with rapeseed oil, which is liquid at 20 °C and rich in unsaturated fatty acids. Both oils are made mostly of triglycerides. A food company wants to make a spread that stays soft when it is stored in a refrigerator.

   (a)

   Describe the structure of a triglyceride and explain how it is synthesised from its components.

   [6 marks]

   (b)

   Explain the difference in physical state between the two oils at 20 °C, and suggest which type of fatty acid the company should choose for the spread.

   [6 marks]

   Total for question 4: 12 marks

End of questions