Safety. Although the dangers of the following experiments are negligible, it is recommended that you consult the latest edition of ‘Safeguards in the School Laboratory’ published by The Association for Science Education (ase.org.uk) before embarking on any experiment.

Scheme. Catalase is an enzyme that is produced in the cells of many living organisms. Some of the energy release reactions in the cell produce hydrogen peroxide as the end product. This compound, which is toxic to the cell, is broken down into water and oxygen by the action of catalase. 2H2O2 = 2H2O + O2

The liver and yeast samples are dropped into hydrogen peroxide. Oxygen is developed and the student is asked to extend the experiment to try to decide if an enzyme in the tissues is responsible. The experiments and questions take about an hour.

Prior knowledge. The existence of inorganic catalysts; enzymes denatured by boiling; oxygen ignites a glowing splint.

Advance preparation and materials – per group

20 volume hydrogen peroxide 50 cm3

pull apart

liver, about 1 cubic cm

distilled water 20 cm3

dry yeast about 1 g

clean sand about 1 g

activated charcoal granules, about 1 g

Device – per group

test tube rack and 4 test tubes

forceps or finder to push the liver into the test tube

4 Gold Spirit Marker Tags

filter funnel

Bunsen lighter

filter paper

test tube holder

mortar and pestle

Experiment

The investigation below is a fairly critical examination of plant and animal tissues to see if

they contain catalase.

(a) Label three test tubes 1-3.

(b) Pour approximately 20 mm (depth) of hydrogen peroxide into each tube.

(c) Cut the liver into 3 pieces.

(d) To tube 1 add a small piece of liver and to tube 2 add a pinch of dry yeast.

(e) Insert a glow ferrule into tubes 1 and 2, bringing it close to the surface of the liquid or on top of the foam.

1 Describe what you saw happen and the effect on the shiny splint.

2 How do you interpret these observations?

3 Is there any evidence from this experiment so far to indicate whether the gas is coming from hydrogen peroxide or from the solid?

4 Is there any evidence at this stage that an enzyme is involved in the production of gas in this reaction?

(f) In tube 3 place some charcoal granules and observe the reaction.

5 Could charcoal be an enzyme? Explain your answer.

6 Assuming (i) that the gas in (f) is the same as before and (ii) that the coal is nearly pure carbon, does the result with coal help you decide on the source of the gas in this experiment and in the other experiments? previous? ?

(g) Suppose it is hypothesized that there is an enzyme in the liver and yeast that breaks down hydrogen peroxide into oxygen and water; Design and carry out a control experiment to test this hypothesis.

7 Record (i) the experiment, (ii) the reasons that led you to carry it out, (iii) the

results and (iv) its conclusions.

(h) Wash the test tubes. Design and carry out an experiment to see if the putative enzyme in plant and animal material can be extracted and still retain its properties. The experiment must include a control.

8 Briefly describe your procedure, your results, and your conclusions.

9 Assuming that liver and yeast each contain an enzyme that breaks down hydrogen peroxide, is there any evidence to show that it is the same enzyme? What would have to be done to know for sure?

Discussion-responses

1 Effervescence should be observed in each case but it is more vigorous with yeast than with liver. The glowing splint should come back on.

2 Oxygen is being produced.

3 There is no evidence to indicate whether the liquid or solid is giving off the gas. If students think a solid is unlikely to give off a gas, they might recall marble and hydrochloric acid where carbon dioxide is produced by the solid. It seems less likely, however, that both yeast and liver emit oxygen when treated with hydrogen peroxide than that hydrogen peroxide must provide oxygen when treated with various substances.

4 So far, there is no evidence that an enzyme is involved.

5 A gas will come out, but not fast enough to relight a glow ferrule. coal could not

be an enzyme because (a) it is an element and (b) it has been produced by very high temperatures that would destroy enzymes.

6 Carbon, as an element, could not be giving off oxygen. The gas must come from

hydrogen peroxide.

7(i) The experiment should involve boiling the tissues and then putting them in hydrogen peroxide.

(ii) If an enzyme is involved,

(iii) no gas will be produced.

8 The student must grind the samples with a little sand and distilled water, filter and test the filtrate with hydrogen peroxide. Oxygen will evolve with a vigor proportional to that observed when the original substances were tested.

The student must boil half of each extract and demonstrate that it loses its activity.

9 There does not appear to be a fundamental reason why yeast and liver should not have different enzymes that catalyze the breakdown of hydrogen peroxide. To be certain on this point, the enzymes would have to be extracted and their chemical composition determined.