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Year 12 Biology Unit 1 – Enzymes Name: INTRODUCTION TO ENZYMES 1. Enzymes are catalysts which catalyse specific reactions by lowering their activation energy. The lock and key and the induced fit models have been used to explain the way in which enzymes work. (a) Explain what is meant by activation energy. ..................................................................................................................................... ..................................................................................................................................... (1) (b) (i) Describe how the lock and key model can be used to explain how an enzyme breaks down a substrate molecule. ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... (3) (ii) Describe how the induced fit model differs from the lock and key model of enzyme action. ........................................................................................................................... ........................................................................................................................... (1) Catalase is an enzyme found in many cells. It catalyses the breakdown of hydrogen peroxide as shown in the equation. hydrogen peroxide catalase water + oxygen Cylinders of potato were cut with a cork borer. The cylinders were then sliced into discs of the same thickness and put into a small beaker containing 50 cm3 of hydrogen peroxide. The mass of the beaker and its contents was recorded over a period of 15 minutes. The results are shown in the graph opposite (c) Explain why the mass of the contents of the beaker fell as the reaction progressed. 100 99 Mass of contents 98 of beaker as percentage of 97 initial mass 96 ....................................................... ........................................................ ................………………………… ………………………………….... 95 0 5 10 Time/minutes 15 (1) Year 12 Biology Unit 1 – Enzymes (d) Name: Explain, in terms of collisions between enzyme and substrate molecules, why the rate of the reaction changed over the period of time shown on the graph. (2) (Total 8 marks) 2. The turnover number of an enzyme is defined as the number of substrate molecules converted to product by one molecule of enzyme in one minute. In an experiment carried out at 20°C, the turnover number for an enzyme was found to be 2500 at the start of the experiment but dropped to 1000 after 5 minutes. (a) (i) Suggest why the turnover number decreased after 5 minutes. ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... (2) (ii) How would you expect the turnover number to differ from 2500 at the start of an identical experiment but carried out at 30°C? Explain your answer. ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... (2) (b) Explain why it would be important to have a control in the experiment at 20°C and at 30°C. ..................................................................................................................................... ..................................................................................................................................... (1) (Total 5 marks)