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The action of catalase on hydrogen peroxide.

Extracts from this document...

Introduction

Biology Assessed Practical - Rosalind Brock Spring 2002 The action of catalase on hydrogen peroxide Aim The aim of this experiment is to discover the relationship between the concentration of the substrate and the rate of the reaction catalysed by an enzyme, by looking at the decomposition of hydrogen peroxide under the action of catalase, and to determine a value for Vmax and the Michaelis constant for catalase. Background Theory An enzyme is a protein biological catalyst. Catalysts speed up or slow down the rate at which chemical reactions occur. They are not used up in the reactions and can be retrieved unchanged afterwards. Biological catalysts control the rate of reactions in living things. Each enzyme is substrate specific - it can control only one reaction. For example, the digestion of starch is begun in the mouth by the enzyme amylase. An equation for this reaction can be shown like this: Amylase Starch ????? Simple sugars The enzyme only facilitates the reaction, it is not used up. Each molecule of enzyme can be reused indefinitely, unless it is damaged, or denatured. Enzymes are proteins, so they are denatured if the polypeptide chains, which are precisely coiled and folded to form the active site, become unfolded by the kinetic energy from heat, or the covalent bonds are disrupted. Whilst some heat will increase the rate of reaction because of the increased number of collisions between enzyme and substrate, too much heat will denature the enzyme and render it completely ineffective. Enzymes are also affected by the pH at which they have to work. Charged hydrogen or hydroxide ions in acids or alkalis can cancel out the charges on the active sites of the enzymes, and render them ineffective. ...read more.

Middle

Variables Independent Variable:- concentration of hydrogen peroxide Dependent Variable:- volume of oxygen gas collected in 15s Controlled Variables:- temperature, volume of hydrogen peroxide, amount of yeast, apparatus, time. * The reaction will be carried out in a water bath at 20?C. Since water is a good thermal buffer it should be fairly easy to keep the temperature constant. * Volume of hydrogen peroxide solution will be controlled quite easily by using two syringes to measure the water and hydrogen peroxide volumes as dictated by the dilution table below. * The amount of yeast will be controlled by making up a suspension of 5g of freeze dried yeast in 50cm3 of water, and frequently stirring the suspension.The yeast will be measured out with a 1cm3 syringe accurate to 0.05cm3. * The apparatus will be kept the same for each solution, and the boiling tube will be rinsed and dried between each reaction. * The oxygen will be collected over water for 15 seconds each time, timed using a stopwatch. Six different concentrations of hydrogen peroxide will be used: 5, 7.5, 10, 12.5, 15, 17, 19 and 20%. More readings are being taken at the higher concentrations to enable the asymptote of the graph to be more easily observed. Three close results (within 10% of each other) will be obtained and averaged to ensure reliable results. Anomalous results (those not which do not fit the general trend and are more than 10% different from the other set) will be repeated until three close results are found, and anomalies will be excluded from the average. The most accurate available and practical equipment will be used in order to ensure precision in measurement. ...read more.

Conclusion

If any results were very different to the rest for that concentration, I excluded them from the average and repeated the experiment again. These results are ringed in red. This could have been caused by any of the errors accounted for in the table above; particularly the difficulty in controlling the amount of yeast. The standard deviations of my results are shown below. % Concentration Hydrogen Peroxide Rate of reaction (cm3min-1) Standard Deviation Set 1 Set 2 Set 3 Average 5 20.80 24.60 25.00 23.47 2.32 7.50 55.00 54.20 63.60 57.60 5.21 10.00 67.60 64.20 84.00 71.93 10.59 12.50 85.20 79.60 96.20 87.00 8.45 15.00 92.80 91.80 102.60 95.73 5.97 17.00 100.80 99.60 110.00 103.47 5.69 19.00 78.80 105.60 112.80 109.20 5.09 20.00 107.20 109.21 114.60 110.34 3.83 These standard deviations are fairly small.. This shows that the averages I obtained were quite reliable. My graph shows a clear trend which enables me to draw an accurate conclusion, but it does not extend far enough to see the asymptote which is predicted by Michaelis-Menton kinetics. There would be two ways of obtaining more accurate values for Vmax and hence Km, either by extending the range to make the appearance of the asymptote clearer, or by using a Lineweaver-Burke plot: which is a graph of 1/[S] against 1/rate. The Michaelis Menton equation can be rearranged to show that the y intercept of the Lineweaver-Burke plot is 1/Vmax, and the gradient of the straight line is Km/Vmax. This is much more accurate than trying to obtain values by determining the value of the asymptote by eye. Better values for the initial rates of reaction could have been obtained if more time were available by tracking the progress of each reaction and drawing a graph of volume of oxygen produced against time, and then taking the gradient at t = 0 for each reaction - the true initial rate. ...read more.

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