Factors affecting enzyme activityAQA A-Level Biology: Subtopic test

1. 1
   A student investigates the effect of temperature on the rate of the reaction catalysed by catalase in potato tissue. She adds 5 g of potato tissue to 20 cm³ of hydrogen peroxide solution at each temperature and collects the oxygen produced for 60 seconds. The volumes of oxygen collected at 20 °C, 30 °C, 40 °C, 50 °C and 60 °C were 4.0 cm³, 9.0 cm³, 15.0 cm³, 3.5 cm³ and 0 cm³ respectively.

   (a)

   Which is the independent variable in this investigation?

   [1 mark]

   • AThe volume of oxygen collected
   • BThe concentration of hydrogen peroxide solution
   • CThe temperature
   • DThe time for which the gas is collected

   (b)

   What was the mean rate of reaction at 30 °C?

   [1 mark]

   • A0.15 cm³ s⁻¹
   • B6.7 cm³ s⁻¹
   • C0.015 cm³ s⁻¹
   • D540 cm³ s⁻¹

   (c)

   Explain why less oxygen was collected at 50 °C than at 40 °C.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Pepsin is a protease secreted into the stomach, where the hydrogen ion concentration of the gastric juice is 6.3 × 10⁻³ mol dm⁻³. Its optimum pH is about 2. Trypsin is a protease secreted into the small intestine, where the pH is about 8, and its optimum pH is about 8.

   (a)

   What is the pH of the gastric juice?

   [1 mark]

   • A2.8
   • B3.2
   • C-2.2
   • D2.2

   (b)

   Which statement best explains why a change in pH alters the activity of an enzyme?

   [1 mark]

   • AHydrogen ions are used up as a substrate
   • BChanges in hydrogen ion concentration affect the ionic and hydrogen bonds that hold the tertiary structure, so the active site changes shape
   • CChanges in pH break the peptide bonds in the primary structure within seconds
   • DChanges in pH alter the activation energy of the uncatalysed reaction

   (c)

   Explain why pepsin is inactive when it passes into the small intestine.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Succinate dehydrogenase catalyses the oxidation of succinate in respiration. Malonate has a molecular shape very similar to succinate and reduces the rate of this reaction. A student also tests inhibitor Q, which binds to a site on the enzyme away from the active site. With malonate present, increasing the succinate concentration to a very high value raises the rate almost to the rate with no inhibitor. With inhibitor Q present, the rate stays well below the rate with no inhibitor, even at very high succinate concentration.

   (a)

   Explain how malonate reduces the rate of the reaction catalysed by succinate dehydrogenase.

   [3 marks]

   (b)

   Explain the different effects of increasing succinate concentration on the rate of reaction with malonate and with inhibitor Q.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student investigates how the concentration of hydrogen peroxide affects the rate of the reaction catalysed by catalase. For each concentration she adds 1.0 g of chopped liver to 10 cm³ of hydrogen peroxide solution in a conical flask at room temperature, seals the flask and collects the oxygen in an upturned measuring cylinder over water for 30 seconds. She carries out one run at each concentration. The volumes of oxygen collected in 30 seconds at 0.20, 0.40, 0.60, 0.80, 1.00 and 1.20 mol dm⁻³ hydrogen peroxide were 3.0, 6.0, 9.0, 10.5, 10.5 and 10.5 cm³ respectively.

   (a)

   Describe and explain the effect of substrate concentration on the rate of reaction, and predict the effect of doubling the concentration of catalase.

   [6 marks]

   (b)

   Evaluate the method used by the student and suggest improvements that would make her conclusions more valid and reliable.

   [6 marks]

   Total for question 4: 12 marks

End of questions