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The Effect of pH on Catalase

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The Effect of pH on Catalase Hypothesis: The rate at which catalase catalyses the decomposition of hydrogen peroxide increases to an optimum pH and then decreases as the optimum pH is exceeded. Biological knowledge: Hydrogen peroxide is produced in plant and animal cells as a by-product, which is toxic, resulting in the need of catalase to speed up the decomposition of hydrogen peroxide to water and oxygen. All microbodies contain catalase and it is the fastest known enzyme with a turnover number of 6 million. It is especially abundant in plant storage organs and in liver. It is effective over the pH range of 4.0 to 9.0 with the optimum pH of 7.6. 2H2O2(aq) catalase 2H2O(l) + O2(g) As an enzyme, its rate is affected by various factors, including pH. Changes in pH alter the ionic charge of acidic and basic groups of the enzyme. This therefore distorts the shape of the enzyme, including its active site, where substrates temporarily bind. The efficiency of forming enzyme-substrate complexes is consequently lowered and the rate of activity decreases. However, the enzyme can regain its most proficient shape, if it is not exposed to too extreme pHs, to return to its maximum rate of activity when placed back at the optimum pH. Plan: The volume of oxygen produced during the catalysed break down of hydrogen peroxide can be measured in a graduated gas syringe whilst catalase, using potato, works at various pHs over a certain time. The hydrogen peroxide should be added to the potato suspension in a flask to start the experiment. Readings of the volume of oxygen produced is to be taken at set intervals during the experiment. The pH of the catalase can be kept constant by a pH buffer solution of the required pH. The average rate of decomposition can be calculated and a graph plotted of pH against rate. This can then be analysed for the effect of pH on catalase to prove or disprove the hypothesis. ...read more.


8. Draw up 2 cm3 of hydrogen peroxide from a crystallising dish with a plastic syringe. Only pour out the hydrogen peroxide just before starting the catalytic reaction, as the solution will decompose very quickly in light. 9. Make sure the gas syringe plunger reads zero and bung the boiling tube of potato. Insert the plastic syringe into the top of the bung. 10. Inject the hydrogen peroxide and start the stop clock at the same time. 11. Record the volume of oxygen produced by the reaction every 10 seconds for one minute and then every minute thereafter, until 4 minutes has passed. 12. Repeat the reaction 3 times and take an average of the volume of oxygen produced at each time interval. 13. Repeat the whole experiment with the other different pHs. 14. Plot a graph of volume of oxygen produced against time and also of the rate of reaction against pH. Results: pH 4.5 Time (s) Volume of O2 produced (cm3 ) 1 2 3 10 2 2 2 20 3 2 2 30 3 3 2 40 3 3 3 50 4 4 3 60 4 5 4 120 9 9 7 180 12 13 8 240 15 15 11 pH 5.5 Time (s) Volume of O2 produced (cm3 ) 1 2 3 10 3 2 2 20 3 3 3 30 4 4 4 40 5 5 5 50 6 6 7 60 7 7 8 120 15 13 15 180 20 18 20 240 23 21 22 pH 6.5 Time (s) Volume of O2 produced (cm3 ) 1 2 3 10 3 3 3 20 4 3 4 30 6 4 6 40 8 6 7 50 10 7 9 60 12 8 10 120 21 16 17 180 26 18 21 240 28 20 23 pH 7.0 Time (s) Volume of O2 produced (cm3 ) 1 2 3 10 2 2 2 20 3 3 3 30 4 4 5 40 5 5 7 50 6 6 8 60 7 7 10 120 12 13 17 180 15 17 22 240 18 20 25 pH 7.5 Time (s) ...read more.


This would dilute the concentration of enzyme, therefore decreasing the rate of reaction. As the volume of pH buffer used was small, only 1 cm3, not all the potato may have been covered by the solution so that not the whole sample of potato would have been at the certain pH of the buffer. The potato itself should have been neutral, so that the rate of decomposition of hydrogen peroxide would have increased slightly with the more extreme high and low pH's. The uneven distribution of pH buffer solution also accounts for the large range of volumes obtained for each experiment within each pH. The temperature of the water bath was very difficult to keep constant at 40oC and so would also affect the rate of reaction and explain the large range of results. The effect of temperature on the enzyme has been explained in the conclusion. An electrical water bath, which is able to maintain a constant temperature, can be used instead of heating up one's own. Readings of the gas syringe can only be to the nearest division making it difficult to state exactly what the volume of gas is. As the scale is marked in 1 cm3 divisions, so the volumes of the results has an uncertainty of � 0.5cm3. In drawing up hydrogen peroxide into the small syringe, there were often bubbles of air trapped in them, which could not be removed, and so the volume of solution measured out would have been very slightly under 2 cm3. it has to be made sure that all the air is pushed out of the syringe and the spout is completely submerged in the hydrogen peroxide before drawing it up. More pH's could be tested, so that a more exact curve for the rate against pH can be plotted to see an extra detailed effect of pH on catalase. To determine a more accurate optimum pH of potato catalase, a narrower range of pH's around 6.5 could be used to find the pH with the maximum rate of oxygen produced and therefore the optimum pH. ...read more.

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Here's what a teacher thought of this essay

4 star(s)

This is a very good report on a practical that demonstrates a good understanding of scientific processes.
1. The background knowledge is good but needs referencing.
2. The preliminary test is also good. However, the conclusion from it needs to be readdressed.
3. The language used is appropriate throughout
4. The conclusion section has become confused about the information that should be included.
5. The evaluation is thoughtful and detailed.
**** (4 stars)

Marked by teacher Luke Smithen 01/05/2013

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