| Safety. Although the hazards in the following | | | | experiments? |
| experiments are negligible, you are advised to consult | | | | (g) Suppose the hypothesis is advanced that there is |
| the latest edition of 'Safeguards in the School | | | | an enzyme in the liver and yeast, which decomposes |
| Laboratory' published by The Association for Science | | | | hydrogen peroxide to oxygen and water; design and |
| Education (ase.org.uk) before embarking on any | | | | carry out a control experiment to test this hypothesis. |
| experiment. | | | | 7 Record (i) the experiment, (ii) the reasons which led |
| Outline. Catalase is an enzyme which occurs in the | | | | you to conduct it, (iii) the observedresults and (iv) your |
| cells of many living organisms. Certain of the | | | | conclusions. |
| energy-releasing reactions in the cell produce hydrogen | | | | (h) Wash out the test-tubes. Design and carry out an |
| peroxide as an end-product. This compound, which is | | | | experiment to see if the supposed enzyme in the plant |
| toxic to the cell, is split to water and oxygen by the | | | | and animal material can be extracted and still retain its |
| action of catalase. 2H2O2 = 2H2O + O2 | | | | properties. The experiment should include a control. |
| Samples of liver and yeast are dropped into hydrogen | | | | 8 Describe briefly your procedure, your results and |
| peroxide. Oxygen is evolved and the student is asked | | | | your conclusions. |
| to extend the experiment to try and decide if an | | | | 9 Assuming that liver and yeast each contain an |
| enzyme in the tissues is responsible. The experiments | | | | enzyme which splits hydrogen peroxide, is there any |
| and the questions take about one hour. | | | | evidence to show that it is the same enzyme? What |
| Prior knowledge. The existence of inorganic catalysts; | | | | would have to be done to find this out for certain? |
| enzymes denatured on boiling; oxygen relights a | | | | Discussion - answers |
| glowing splint. | | | | 1 Effervescence should be observed in each case but |
| Advance preparation and materials - per group | | | | it is more vigorous with yeast than with liver. The |
| 20 volume hydrogen peroxide 50 cm3splintliver, about 1 | | | | glowing splint should relight. |
| cm cubedistilled water 20 cm3dried yeast about 1 | | | | 2 Oxygen is being produced. |
| gclean sand about 1 gactivated charcoal granules, | | | | 3 There is no evidence to indicate whether the liquid or |
| about 1 g | | | | solid is giving the gas. If the students think that a solid is |
| Apparatus - per grouptest-tube rack and 4 | | | | unlikely to give off a gas they could be reminded of |
| test-tubesforceps or seeker for pushing liver into | | | | marble and hydrochloric acid in which it is the solid |
| test-tube | | | | producing the carbon dioxide. It seems less likely, |
| 4 labels or spirit markerfilter funnel | | | | however, that yeast and liver would both give off |
| Bunsen burnerfilter papertest-tube holdermortar and | | | | oxygen when treated with hydrogen peroxide, than |
| pestle | | | | that hydrogen peroxide should give oxygen when |
| Experiment | | | | treated with diverse substances. |
| The investigation below is a fairly critical examination of | | | | 4 So far, there is no evidence of an enzyme being |
| plant and animal tissues to see ifthey contain catalase. | | | | involved. |
| (a) Label three test-tubes 1-3. | | | | 5 A gas will come off but not sufficiently rapidly to |
| (b) Pour about 20 mm (depth) hydrogen peroxide into | | | | relight a glowing splint. Charcoal could notbe an |
| each tube. | | | | enzyme because (a) it is an element and (b) it has |
| (c) Cut the liver into 3 pieces. | | | | been produced by very high temperatures that would |
| (d) To tube 1 add a small piece of liver, and to tube 2 | | | | destroy enzymes. |
| add a pinch of dried yeast. | | | | 6 Charcoal, as an element, could not be giving off |
| (e) Insert a glowing splint into tubes 1 and 2, bringing it | | | | oxygen. The gas must be coming from thehydrogen |
| close to the liquid surface or into the upper part of the | | | | peroxide. |
| froth. | | | | 7 (i) The experiment should involve boiling the tissues |
| 1 Describe what you saw happening and the effect on | | | | and then putting them into hydrogen peroxide. |
| the glowing splint. | | | | (ii) If an enzyme is involved, |
| 2 How do you interpret these observations? | | | | (iii) no gas will be produced. |
| 3 Is there any evidence from this experiment so far, to | | | | 8 The student should grind the samples with a little |
| indicate whether the gas is coming from the hydrogen | | | | sand and distilled water, filter and test the filtrate with |
| peroxide or from the solid? | | | | hydrogen peroxide. Oxygen will be evolved with a |
| 4 Is there any evidence at this stage that an enzyme | | | | vigour proportional to that witnessed when the original |
| is involved in the production of gas in this reaction? | | | | substances were tested. |
| (f) In tube 3 place a few granules of charcoal and | | | | The student should boil half of each extract and show |
| observe the reaction. | | | | that it loses its activity. |
| 5 Could charcoal be an enzyme? Explain your answer. | | | | 9 There seems no fundamental reason why yeast |
| 6 Assuming (i) that the gas in (f) is the same as before | | | | and liver should not have different enzymes which |
| and (ii) that the charcoal is almost pure carbon, does | | | | catalyse the decomposition of hydrogen peroxide. To |
| the result with charcoal help you to decide on the | | | | be certain on this point, the enzymes would have to be |
| source of the gas in this and the previous | | | | extracted and their chemical composition determined. |