(Understanding Biology for Advanced Level by Glenn and Susan Toole. Nuffield Biology’)
Only a small amount of enzyme is ever needed to catalyse a large amount of substrate because all activity takes place at the enzyme’s active site and after a reaction has taken place the enzyme is left unchanged. This means that the same enzyme is still available for further reactions.
The predicted rate of reaction is shown on the graph below:
Graph to show the predicted rate of reaction at different temperatures
In the experiment I plan to change only the temperature. All of the other variables will be kept the same so that the experiment is a fair test as to whether temperature affects the rate of enzyme action. These include the volume of the hydrogen peroxide, the length of the potato cylinder and the time in which the experiment is allowed to happen. I chose to use a potato cylinder for this because it would make it easier to ensure that they were all the same length, thus saving time.
To make sure that the experiment is accurate, I shall use the same equipment each time to measure out the size of the potato cylinder and a measuring cylinder which shows units down to 0.2ml to measure the correct amount of hydrogen peroxide.
Also, so that I get an accurate result, I shall repeat each experiment three times and then take an average. This will make sure that any ‘odd’ results do not become part of the experiment, as it should be easy to see whether it is around the right amount of oxygen. This should also help to eliminate any errors that may occur.
Equipment
2 beakers, hydrogen peroxide, liquefied potato, stop-clock, boiling tube, rubber tubing with rubber bung, large measuring cylinder, small measuring cylinder, thermometer, stand, clamp, bowl filled with water
Trial
To get the required amount of catalase, I used potato cylinders of a specific length. Potato cells contain a lot of catalase and this should react with the hydrogen peroxide to break it down into hydrogen and oxygen.
The decomposition of hydrogen peroxide gives off oxygen and it is the volume of this gas that I shall be measuring in the time limit, to find out the rate of the reaction.
For the trial, I used 4ml of hydrogen peroxide and added a cylinder of potato 3cm long. I then timed the experiment for three minutes, to give time for the whole reaction to occur.
I found that this amount of hydrogen peroxide and potato did not produce enough oxygen for me to measure accurately. I therefore decided to increase the volume of hydrogen peroxide. I shall also increase the amount of potato used. This will mean that there will be a larger surface area of the catalase in contact with the hydrogen peroxide, which should help to speed up the reaction. I shall also use 6ml of hydrogen peroxide for each temperature and add it to 2 grams of the liquefied potato. As it has been liquefied, the catalase will be more accessible to the hydrogen peroxide and, therefore, the reaction should happen faster. This is because it will create a larger surface area for the catalase allowing more to be in contact with the hydrogen peroxide at any one time. I shall then time the experiment for 4 minutes so that it will have enough time to react completely.
Method
- Set up the experiment as shown in the diagram below:
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Fill the first beaker up to around half full and heat this up to 20oC.
- Measure out 2 grams of liquidised potato. To make sure that the experiment is accurate, use a set of digital scales that can measure down to two decimal places. This should help to eliminate any anomalous results that may be caused by different amounts of liquidised potato being used. If different amounts were used the experiment would not be a fair test as each of the experiments would be likely to contain different amounts of catalase. This would mean that for each of the experiment the catalase would have a different surface area in contact with the hydrogen peroxide changing the speed at which the reaction should take place. This is one of the factors that must be controlled in the experiment to make sure that it is a fair test. Another factor that must be controlled is the concentration of the hydrogen peroxide. If the concentration were to increase, the amount of oxygen would also increase. This would happen because there would be more of the hydrogen peroxide in contact with the catalase at any one time. If this were to happen over the course of two or more of the tests, it could quite easily change the results. There would be more collisions between the enzyme and the substrate and the substrate would ‘fit’ into the enzyme more often. This would mean that more oxygen would be produced, as more hydrogen peroxide would be broken down. The concentration of the hydrogen peroxide that should be used is 2 moles.
- Place the hydrogen peroxide into the boiling tube and put the tube into the water filled beaker.
- Allow 2 minutes for the hydrogen peroxide to warm up to the required temperature.
- Add the potato to the hydrogen peroxide and seal the top of the flask with the bung so that the oxygen has to travel along the tube and enter the upturned measuring cylinder.
- Time for four minutes and then stop the experiment.
- Measure the amount of oxygen that has collected at the top of the measuring cylinder and record the measurement on the table.
- Repeat this two more times and then take an average of the amount of gas collected.
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Repeat steps 1-8 with temperatures of 30oC, 40oC, 50oC, and 60oC.
- Repeat the experiment three times at each temperature.
Results
As the rate of enzyme action was very small, I decided to scale the number up by a factor of 1,000. This will make it easier to do any calculations that may need to be done.
Conclusion
The graph shows that there is a direct correlation between temperature of the catalase and the amount of rate of the reaction when it is added to hydrogen peroxide. The x-axis shows the temperature of the enzyme for each experiment and the y-axis shows the rate of reaction of the enzyme at each of the temperatures. This was worked out by doing the equation:
Average Volume
Time
This gave the rate of the reaction of the enzyme for each of the different temperatures. It was, therefore, much easier to compare the results than if I had just used the volume of gas produced. The best-fit line shows the correlation between the two factors as it lies along the curve created by most of the points. This shows there to be a direct correlation between the results.
In my prediction I stated that for every ten-degree increase, the rate of reaction would double until the temperature reached 40oC. At this point the temperature would begin to change the shape of the enzyme, stopping the enzyme from catalysing the hydrogen peroxide.
This correlation is due to the catalystic nature of some enzymes. The purpose of most enzymes is to speed up the reaction of other molecules. This can be very useful if it is part of a plant or animal as it can help to break down many of the substances animals eat or plants take in. An example of this in the human body is that of amylase in the mouth. This breaks starch molecules down into maltose. This is a much simpler molecule than starch and it is therefore easier for this to be broken down into glucose.
Catalase is found in potatoes and this enzyme has the property of speeding up the breakdown of hydrogen peroxide (H2O2) into it’s original elements, hydrogen (H) and oxygen (O).
Hydrogen Peroxide Water + Oxygen
Catalase
2H2O2 2H2O + O2
The oxygen given off is the gas that we can measure by forcing it into the measuring cylinder via a rubber tube.
All enzymes work on the process of the lock and key (Induced Fit) theory. The substrate (in this case hydrogen peroxide) forms an exact fit with the enzyme molecule, producing an enzyme-substrate molecule. The hydrogen and oxygen split apart from the hydrogen peroxide molecule, which has now been split into its two separate components. The enzyme is left unchanged and ready for the next hydrogen peroxide molecule to link with its active site. This is how an enzyme can speed up a reaction without being changed itself, the process being illustrated in the diagram below.
The induced fit theory states that the enzyme and substrate molecules do not have to be an exact fit. When the substrate molecule meets the active site of the enzyme, the latter moulds itself into the exact shape of the hydrogen peroxide molecule thus enabling the split of hydrogen and oxygen to take place.
(Understanding Biology for Advanced Level by Glenn and Susan Toole)
The lock and key theory
Temperature affects this process because it gives more kinetic energy to both the substrate and the enzyme. This means that they are much more likely to hit each other as they are moving around faster. For every ten degrees C that is added to a molecule, it’s kinetic energy doubles. This means that for every ten degrees added, the rate of reaction should double as there should be double the amount of collisions between the substrate and the enzyme.
This continues up until around 40oC. At this temperature the enzymes begin to become de-natured and will not have the ability to breakdown the substrate. This happens because all enzymes are made up of protein strands. All proteins are affected by temperature in that they begin to change shape at around 40oC. As enzymes work by the lock and key theory, any changes of shape will stop the enzyme working completely because the substrate will no longer fit into the enzyme.
Evaluation
Analysis of Results
- Most of the results that I got from the experiment formed a good curve on the graph. Only one of the results that I got did not fit properly onto the best-fit line. There are many reasons why this may have occurred.
- One possibility is that the temperature of the enzyme was not at the temperature of the surrounding water. This would mean that when the enzyme should have been at a certain temperature and it wasn’t, this would have given different results.
- Another possibility is that I did not use exactly the same amount of enzyme for each of the different experiments. Not every potato has exactly the same amount of catalase in each of its cells. Although the potato was liquidised, the experiment was carried out over several lessons and each sample used may not have had as much catalase as previous ones. This would mean that there was not as much catalase for the same amount of hydrogen peroxide resulting in less reaction, and so lower oxygen production.
- Another factor that could change the accuracy of the results was the temperature around the measuring cylinder collecting the oxygen. Gases expend when their temperature increases and contract when their temperature decreases. This would mean that if the temperature of the gas surrounding the oxygen were different, the volume of gas would change. This would show up in the results as either a slightly smaller or larger reading than would be expected at that temperature.
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In my prediction, I stated that for every 10o increase, the rate of reaction should double. This did not happen in the experiment. This may have been because the variables shown above were not constant or that the temperature of the water was continually dropping. The result would be less energy in the enzyme and which would react slower.
- This may have happened because during the experiment, a lot of the gas produced was contained within a layer of bubbles on top of the hydrogen peroxide. Much of the gas produced by the experiment would have been held within these bubbles. In that case, not all of the gas produced would have been measured. If not all of the gas was being measured, the results would be seriously changed and less gas would be shown to have been produced by the reaction.
Improving the Experiment
- To improve the experiment, I would have to control the temperature around the measuring cylinder. If the air around the cylinder was colder than that of the oxygen being produced, this would reduce the volume of gas. This would happen because a gas expands as the temperature increases and contracts if the temperature decreases resulting in a different volume of gas collecting at the top of the cylinder, seriously changing the results.
- To improve this further, I could have put the apparatus into a thermostatically controlled water bath. This would mean that the temperature of the enzyme would be kept at exactly the same temperature for all of the experiments. As the reaction itself is exothermic, its own reaction would heat increase its temperature. The water bath would help to control this as any extra heat would be transferred to the water. This would mean that the experiment would show a more accurate analysis of whether temperature affects the rate of enzyme action.
- I could also use a buffer to keep the pH of the hydrogen peroxide the same. During the experiment the reaction would have changed the pH (acidity/alkalinity) of the solution. As enzymes all need a certain pH at which the enzyme can reach its fastest rate of reaction. Any changes in pH would change the rate of reaction, so a buffer would be used to would keep pH constant.
- Whilst I was conducting the experiment, there was a time lapse between when I first added the potato to the hydrogen peroxide, and when I placed the bung with the tube in it onto the boiling tube. To overcome this, I could either syringe the potato into the solution or put it into a test tube which would ‘rot’ and release the enzyme into the hydrogen peroxide. Both of these solutions would stop any oxygen produced being let out before the bung can be added.
- To make the experiment a more accurate reflection of the way temperature affects the rate of enzyme action, I could use catalase extract. During the experiment, the catalase was part of a potato. This would have affected the results because some of the enzyme would have still been trapped within the cells, unable to react. Even though I liquidised the potato, there would still have been a lot of catalase which would not have reacted with the hydrogen peroxide. Using catalase extract would solve this because none of the catalase would be trapped and all of it would be free to react with the hydrogen peroxide.
Extending the Experiment
- To extend the experiment, I could try using different enzymes. In the experiment, I was only testing whether temperature affected the rate of one enzyme, catalase. To find out whether temperature affects all enzymes, then I would have to use many different enzymes or else it would not be a fair test.
- I would also have to test as to whether the enzymes only affected hydrogen peroxide. I would have to use many other substances, such as those found in the human body, to find this out.
- To make sure that there were no ‘freak’ results within the data, I could also repeat the experiment more than three times. This would mean that it would be much harder for any errors to show up in the results.
