Cambridge International A Level Biology Revision Guide

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Cambridge International AS Level Biology 6 The graph shows the effect of changes in pH on the activity of the enzyme lysozyme. Rate of reaction 2 3 4 5 6 pH 7 8 9 10 a Describe the effect of pH on this enzyme. [2] b Explain why pH affects the activity of the enzyme. [4] [Total: 6] 7 The graph below shows the effect of temperature on the rate of reaction of an enzyme. X A B 68 Rate of reaction 0 10 20 30 40 50 60 Temperature / °C a What is indicated by X? [1] b What temperature would X be for a mammalian enzyme? [1] c Explain what is happening in region A. [3] d Explain what is happening in region B. [3] e Enzymes are effective because they lower the activation energy of the reactions they catalyse. Explain what is meant by ‘activation energy’. [2] [Total: 10] 8 The reaction below occurs during aerobic respiration. The reaction is catalysed by the enzyme succinate dehydrogenase. a Name the substrate in this reaction. [1] COOH CH 2 CH 2 succinate dehydrogenase COOH CH CH COOH succinic acid COOH fumaric acid
Chapter 3: Enzymes b c The molecule malonic acid, which is shown here, inhibits this reaction. COOH It does not bind permanently to the enzyme. Describe how malonic acid inhibits the enzyme succinate dehydrogenase. CH 2 [3] Heavy metals such as lead and mercury bind permanently to –SH groups COOH of amino acids present in enzymes. These –SH groups could be in the active site or elsewhere in the enzyme. malonic acid i Name the amino acid which contains –SH groups. [1] ii Explain the function of –SH groups in proteins and why binding of heavy metals to these groups would inhibit the activity of an enzyme. [4] iii What type of inhibition would be caused by the heavy metals? [1] [Total: 10] 9 You are provided with three solutions: A, B and C. One solution contains the enzyme amylase, one contains starch and one contains glucose. Starch is the substrate of the enzyme. The product is the sugar maltose. You are provided with only one reagent, Benedict’s solution, and the usual laboratory apparatus. a Outline the procedure you would follow to identify the three solutions. [6] b What type of reaction is catalysed by the enzyme? [1] [Total: 7] 10 The activity of the enzyme amylase can be measured at a particular temperature by placing a sample into a Petri dish containing starch-agar (‘a starch-agar plate’). Starch-agar is a jelly containing starch. One or more ‘wells’ (small holes) are cut in the agar jelly with a cork borer, and a sample of the enzyme is placed in each well. The enzyme molecules then diffuse through the agar and gradually digest any starch in their path. At the end of the experiment, iodine in potassium iodide solution is poured over the plate. Most of the plate will turn blue-black as iodine reacts with starch, but a clear ‘halo’ (circle) will be seen around the well where starch has been digested. Measuring the size of the halo can give an indication of the activity of the enzyme. A student decided to investigate the rate at which a mammalian amylase is denatured at 60 °C. She heated different samples of the enzyme in a water bath at 60 °C for 0, 1, 5, 10 and 30 minutes. She then allowed the samples to cool down to room temperature and placed samples of equal volume in the wells of five starch-agar plates, one plate for each heating period. She then incubated the plates in an oven at 40 °C for 24 hours. The results of the student’s experiment are shown on the next page. A diagram of one dish is shown, and the real size of one halo from each dish is also shown. a Why did the student cut four wells in each dish rather than just one? [1] b One dish contained samples from amylase which was not heated (time zero). This is a control dish. Explain the purpose of this control. [1] c Explain why the starch-agar plates were incubated at 40 °C and not room temperature. [1] d Describe what was happening in the dishes during the 24 hours of incubation. [4] e Why was it important to add the same volume of amylase solution to each well? [1] f Measure the diameter in mm of the representative halo from each dish. Record the results in a suitable table. [4] g Only one halo from each dish is shown in the diagrams. In practice there was some variation in the diameters of the four halos in each dish. How would you allow for this when processing your data? [1] h Plot a graph to show the effect of length of time at 60 °C on the activity of the enzyme. [5] i Describe and explain your results. [4] continued ... 69
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Mary Jones, Richard Fosbery, Jennif
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Cambridge International AS Level an
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Chapter 11: Immunity Smallpox was f
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Chapter P2: Planning, analysis and
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Appendix 2: DNA and RNA triplet cod
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Glossary artery a blood vessel that
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Index gluconeogenesis, 317 glucose,
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Index pacemaker, 177 palisade mesop
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Index tar, 190, 191, 193 taste buds
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Acknowledgements Meiosis” in Tuat
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Cambridge International AS and A Le
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Cambridge International A Level Biology Revision Guide

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