Composition of citric acid-sodium phosphate buffer solutions at various pHs
Method
- Set up apparatus as shown in diagram below.
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Place 3 cm3 of minced potato in the conical flask and add 10 cm3 of buffer solution. Swirl gently to mix.
- Replace the bung in the flask ensuring an airtight seal. Fill the graduated tube with water and invert it carefully into the beaker of water. Do not place over the end of the delivery tube.
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Measure 5 cm3 of hydrogen peroxide into the syringe and connect to the flask. Inject the hydrogen peroxide. This will displace air, which must not enter the graduated tube.
- Immediately position the graduated tube over the end of the delivery tube and measure the volume collected every 30 seconds for 5 minutes.
- Repeat the experiment using a different buffer solution and fresh potato.
Apparatus
Results and Discussion
- The Results table can be found below.
The graph of these results is the following:
Two observations were made from the above graph:
(a) From the graph, it can be deduced that at the pH level 6.5, the catalase was most active as it had the fastest rate of oxygen gas collected. From the graph it could be said that 6.5 pH is the optimum environment for the catalase enzymes to survive at.
(b) By looking at the graph, it is obvious that at the pH level 4.4, the catalase was least active as it took the longest time to react and it still didn’t get to the point where 15cm3 of oxygen was collected.
Although the graph shows that 6.5 pH is the optimum environment, it is not very accurate as it could be between 6.5 and 7.5 pH. Therefore to obtain this accurate result, the data for the rates of reactions for the first 90 seconds was plotted on the graph below. (After the results table from the first 90 seconds.)
The above graph shows that the optimum pH level is around pH 6.6, this is seen where all three curves peak and it is where there is a dotted line drawn on the graph.
A number of things were kept constant in this experiment, they are: pH levels, the reaction was left for the same length of time and the volumes of the minced potato and the solutions.
One thing that would have affected the results is the varying temperature, this is due to the reaction being exothermic and therefore the temperature in the flask would rise. This would, therefore, increase the rate of reaction. To overcome this problem, a water bath could be used to keep the temperature constant. Another thing is, that it was difficult to obtain exactly 3cm3 of minced potato. This is because when using the syringe, the potato kept getting stuck on the syringe. By using a more suitable method of getting all the potato into the flask, this problem can also be overcome.
Each individual enzyme has an optimal pH range that helps to maintain its normal configuration in the environment in which it operates. From the experiment, catalase was found to have an optimum temperature of 6.6pH. The tertiary structure of a protein depends on interactions such as hydrogen bonding, between R groups. A change in pH can alter the ionisation of these side chains and disrupt the normal configuration and in some cases denature the enzyme. A denatured protein cannot combine with a substrate.