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To examine the effect of Substrate Concentration (Hydrogen Peroxide) on the rate of an enzyme catalysed reaction

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Introduction

Kevin McAllister To examine the effect of Substrate Concentration (Hydrogen Peroxide) on the rate of an enzyme catalysed reaction The reaction we will study is: 2H2O2 ? O2 + H2O Catalase Enzymes are globular proteins with a complex tertiary structure. Weak chemical bonds between the polypeptide chains maintain the shape of the enzyme, which is critical to its function. Each enzyme has a specifically shaped active site. Substrate molecules which have the same shape as the active site may combine with this active centre - forming an enzyme substrate complex. The enzyme and substrate molecules interact in such a way that a chemical reaction occurs and appropriate products are produced. This sequence of events can be repeated very rapidly. Catalase is a good example, it occurs naturally in the body tissues of plants and animals and catalyses the decomposition of Hydrogen peroxide to oxygen gas and water by up to 10,000 times. Substrate Concentration has a major effect on the rate of reaction. At low substrate concentrations many of the available enzyme molecules will have active centres which are unoccupied and the restricted supply of substrate molecules will mean less enzyme substrate complex's will form per unit of time, so the rate of reaction will be low. As we increase substrate concentration more of the active centres will be occupied so more enzyme substrate complexes will form per unit of time and therefore rate of reaction will increase. When large concentrations are used the rate of reaction will increase no longer as all the enzymes active sites' are filled so rate of reaction will not be able to increase further until concentration of enzyme is increased. This is shown by the graph below. Effect of substrate and enzyme concentration on the rate of an enzyme controlled reaction. We have chosen to use the effect of Catalase enzyme on Hydrogen Peroxide because this is a very easy system to use. ...read more.

Middle

The system can be checked by filling the syringe with 10 cm3 of water and squirting it into the conical flask through the bung and tube. The gas syringe should then show a reading of 10 cm3 if there is no gas escapage from the system. If the gas syringe does not show this reading then the system should be checked again to check that no gas can escape. 1.10g of potato will be weighed on a top pan balance accurate to 2 decimal places. It will be cut using a potato corer to ensure a constant surface area and placed into the conical flask. Using the graduated pipette, 10 cm3 of Hydrogen Peroxide should be obtained and placed into the syringe. Check that the Gas syringe is set at 0. Apply pressure to the syringe containing the Hydrogen Peroxide until all the contents have been added to the conical flask. Immediately start the stopwatch. Take readings of oxygen evolution from the gas syringe every 20 seconds and record them in a table. After 180 seconds stop recording results. The entire system should then be thoroughly washed and dried, as when the experiment is repeated at different substrate concentrations if any solution from the previous experiment remains on the apparatus then the concentrations will be affected, and the test will be unfair. The above steps can then be repeated using the concentrations of Hydrogen Peroxide that have already been stated. Three sets of results should be obtained for each substrate concentration. Apparatus Bench mat 10 volume Hydrogen Peroxide Goggles Water Conical Flask Boss and Clamp Graduated Pipette Stand Bung and Delivery Tube Gas Syringe 2 X Syringe I calculated gradients for the graphs I produced of my results. The gradients were all taken from the same position on the graph so they can adequately allow a comparison of the rates of reaction of different substrate concentrations. ...read more.

Conclusion

The balance we used to weigh the potato was only accurate to two decimal places so there could have been an error of 0.625% (0.01/1.6) x 100 = 0.625% A graduated pipette is only accurate to around 0.25%, and the syringe we used to add the enzyme to the system was completely inaccurate and we had to calibrate it by using the graduated pipette. The gas syringe sometimes got stuck, and therefore oxygen being produced was not being recorded. Although we wished to use the same potato to obtain all our samples from, this was not possible. We had to use different potatoes as one potato could not provide us with enough tubes. Different potatoes could have different concentrations of enzymes within them and also they could be of different ages, this could affect the rate of the reaction. When the substrate was added to the system, although 10cm3 of Hydrogen peroxide had been added to the system, sometimes the gas syringe showed a reading of less than this value. This was because the gas syringe required the pressure within the system to be sufficient before it would be pushed out. The gas which is unaccounted for has been used to increase the systems' pressure so that the gas syringe will operate. We could also not tell how far the Hydrogen peroxide had decomposed. If we were to run the experiment again, I would first run a 10cm3 sample of the 10-volume hydrogen peroxide with a high catalase concentration over a period of about an hour in a closed system. This would show us how much Oxygen was being produced, and because we know 100cm3 of oxygen should be produced we could calculate the purity of the Hydrogen peroxide. I would then obtain results over a wider range of concentrations, to see if the conclusions I have drawn from this small set of results are consistent over a wider range of samples. I would also investigate to see if the same conclusions can be applied if the substrate concentration is the constant factor and the enzyme concentration varied, as in theory they should. ...read more.

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