Chapter 4 - Enzymes and Metabolism (CE Bio Structured Questions).doc

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HKDSE Biology – Past Examination Questions (Structured Questions) Chapter 4 – Enzyme and Metabolism BIO-CE-1993 I Q1a In an experiment to study the activity of catalase in potato tubers, a 2 cm long cylinder of potato tissue was prepared using a cork borer. The cylinder was cut into 10 discs which were put into the barrel of a syringe. Then 10 cm 3 of dilute hydrogen peroxide solution was drawn into the barrel and the mouth of the barrel was sealed by a rubber cap. The gas released was collected as shown in the diagram below: The volume of gas collected in the barrel was recorded throughout the experiment as shown in the table below: (i) Present the result of the experiment in the form of a graph. (3 marks) (ii) At which time interval was the reaction rate (1) the highest? (1 mark) _______________________________________________________________________________________________________ (2) the lowest? (1 mark) _______________________________________________________________________________________________________ (iii) Explain the changes in the reaction rate throughout the experiment. (4 marks) 1
5 4 3 2 1 0 5 1 0 1 5 2 0 2 5 3 0 V o l u m e o f g a s c o l l e c t e d ( c m ) Time (min) 3 _______________________________________________________________________________________________________ _______________________________________________________________________________________________________ _______________________________________________________________________________________________________ _______________________________________________________________________________________________________ _______________________________________________________________________________________________________ _______________________________________________________________________________________________________ _______________________________________________________________________________________________________ _______________________________________________________________________________________________________ (iv) On the same graph that you have plotted in (i), sketch a curve to show the volume of gas likely to be collected throughout the experiment if 20 potato discs had been used instead of 10. (2 marks) Total: 11 marks BIO-CE-1993 I Q1a Marking Scheme (i) Correct choices of axes (A) and correct labelling of axes (L) 1, 1 Correct plotting and joining of points and straight-lined (P) 1 (axes without scale, P = 0) (not on graph paper, P=0) (ii) (1) 0 - 2 minutes 1 (2) 20 - 30 minutes 1 (iii) As the reaction processed, the reaction rate decreased 1 because the substrate concentration was decreasing 1 thereby reducing the chance of collision between the substrate and enzyme molecules 1 When all the substrate was used up, the reaction stopped 1 (iv) The curve is to the left of that plotted in (i) 1 The curve levels off at 5 cm 3 of gas collected 1 BIO-CE-1993 I Q1a Examination Report (i) Most candidates were able to choose and label the axes correctly and plot the points accurately. A small number of 2
candidates, however, plotted their graphs in their answer books instead of on the graph paper. (ii) Some left out the unit (i.e. minutes) for the time interval, (iii) This part was satisfactory, but the following errors were frequently made: 1. Some candidates did not understand that the slope of the curve represented the reaction rate. They equated the volume of gas collected with the reaction rate. Consequently, they wrongly stated that the reaction rate increased with time as more and more gas was collected. 2. Some candidates were able to state that the reaction rate decreased with time, but gave the wrong explanation, e.g. that the decrease in reaction rate was due to a drop in the amount of catalase. This reflected the fact that they held the serious misconception that enzyme molecules were used up during a reaction. 3. Most candidates could not relate the decrease in reaction rate to the reduced chance of collision between the substrate and enzyme molecules as the concentration of the substrate molecules decreased . 4.

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