EnzymesEdexcel International A Level Biology: Flashcards

Exam questions on Enzymes

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.
   Explain why catalase breaks down hydrogen peroxide but does not break down other substances such as glucose.2 marks
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.
   Explain why the rate of reaction increases between 20 °C and 40 °C.2 marks
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.
   Explain why pepsin has almost no activity at pH 8.3 marks