EnzymesEdexcel International A Level Biology: Subtopic test

1. 1
   Catalase is an enzyme found inside the cells of many organisms. It catalyses the breakdown of hydrogen peroxide, a toxic by-product of metabolism, into water and oxygen: 2H₂O₂ → 2H₂O + O₂. Without catalase, hydrogen peroxide breaks down very slowly at body temperature.

   (a)

   How does catalase increase the rate of breakdown of hydrogen peroxide?

   [1 mark]

   • AIt increases the energy of the hydrogen peroxide molecules
   • BIt provides a reaction pathway with a lower activation energy
   • CIt supplies energy from ATP to the reaction
   • DIt raises the temperature of the cell

   (b)

   Which of these enzymes is an extracellular enzyme?

   [1 mark]

   • AAmylase secreted by the salivary glands into the mouth
   • BCatalase in the peroxisomes of liver cells
   • CDNA polymerase in the nucleus
   • DATP synthase in mitochondria

   (c)

   Explain why catalase breaks down hydrogen peroxide but does not break down other substances such as glucose.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student investigates the effect of temperature on the activity of amylase. At each temperature she mixes amylase with starch solution and records the time taken for all the starch to be broken down, shown by iodine solution no longer turning blue-black. At 20 °C it took 240 s, at 30 °C it took 120 s, at 40 °C it took 60 s and at 50 °C it took 90 s. At 60 °C the starch was still present after 10 minutes.

   (a)

   What is the rate of reaction at 30 °C, calculated as 1/time?

   [1 mark]

   • A120 s⁻¹
   • B0.12 s⁻¹
   • C0.017 s⁻¹
   • D8.3 × 10⁻³ s⁻¹

   (b)

   Which statement best explains why the starch was not broken down at 60 °C?

   [1 mark]

   • AThe amylase molecules had too little kinetic energy to collide with starch
   • BAll the starch was used up in the first few seconds
   • CThe amylase was denatured, so its active site no longer fitted the starch
   • DThe high temperature broke the peptide bonds in the primary structure of the enzyme

   (c)

   Explain why the rate of reaction increases between 20 °C and 40 °C.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Pepsin is an enzyme secreted into the stomach, where the pH is about 2. Trypsin is secreted into the small intestine, where the pH is about 8. Pepsin has an optimum pH of about 2 and almost no activity at pH 8. Both enzymes digest proteins in the lumen of the gut, outside the cells that made them.

   (a)

   Explain why pepsin has almost no activity at pH 8.

   [3 marks]

   (b)

   Explain why it is an advantage for digestive proteases such as pepsin to be extracellular, whereas an enzyme such as catalase is intracellular.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student investigates the effect of substrate concentration on the initial rate of the reaction catalysed by catalase. She uses the same mass of catalase solution each time and collects the oxygen produced from hydrogen peroxide solutions of different concentrations. The volumes of oxygen collected in the first 30 seconds at hydrogen peroxide concentrations of 0.2, 0.4, 0.6, 0.8, 1.0 and 1.2 mol dm⁻³ were 3.0, 5.5, 7.4, 8.4, 8.6 and 8.7 cm³ respectively.

   (a)

   Describe and explain the pattern in the results, including a calculation of the initial rate at 0.6 mol dm⁻³.

   [6 marks]

   (b)

   The student repeats the investigation with twice as much catalase. Predict and justify the effect on the rate at 1.0 mol dm⁻³ and on the shape of the curve.

   [6 marks]

   Total for question 4: 12 marks

End of questions