I shall not mix the different concentrations, especially because the concentration is my dependent variable. I can do this by labeling beakers clearly, clean apparatus after use of the individual concentrations.
Reliable results: -
I want reliable results so I shall do each different concentration three times. I will then check that the results for each concentration to see if they are similar, if there is an anomalous result I shall find out why and find out what happened to cause it and repeat the experiment.
I will also use a thermometer so that I can check that temperature is constant, as a difference in temperature could affect my results.
Prediction: -
My prediction is the lower the concentration of hydrogen peroxide against the same sized piece of potato cylinder the less oxygen will be produced and in contrast the higher the concentration the more oxygen should be produced.
My reasons for this is as follows- The higher the concentration of hydrogen peroxide the greater the number of hydrogen peroxide molecules per cm³will be present to be broken down by the enzyme molecules, so more oxygen will be produced.
Thus, the lower the concentration, fewer molecules per cm³ will be present to be broken down by the enzyme molecules equaling less oxygen being produced.
Quantative Prediction: -
In order to react, molecules need to be able to move and collide with each other and the more they collide the more oxygen will be produced. There are ways to increase the Kinetic Energy the molecules need to move. Increasing the temperature to the experiment’s optimum, enabling the molecules to collide more frequently, thus increasing the rate of the reaction; i.e. more Oxygen will be produced in one minute if the temperature is increased; this is why I am monitoring the temperature as it is not the effect of temperature on the rate of the reaction that I want to test.
I am going to increase the concentration of the Hydrogen Peroxide from 0.25M-2.0M; this will give a greater chance of the Hydrogen peroxide molecules hitting an active site. If I was to double the concentration, I am doubling the amount of Hydrogen Peroxide molecules per cm³which will then double the number of collisions between the enzymes and the hydrogen peroxide molecules so in theory; I will double the rate of the reaction (amount of oxygen produced/min) by doubling the concentration of Hydrogen Peroxide, because twice as many hydrogen peroxide molecules will be available to be broken down.
Preliminary Experiment: -
When I did my preliminary experiment I tested for: -
- The length of potato cylinder I should use.
- The amount of hydrogen peroxide I would need.
- My range of concentration of hydrogen peroxide.
Potato Cylinders: -
The lengths of the potato cylinders are 5cm, but found that the amount of Hydrogen Peroxide I used did not completely surround the potato, which would restrict the surface area and concentration of Catalase it was reacting with. I shall cut each one up into five equal 1cm pieces, giving a greater surface area. I will do this so that the more enzymes will be exposed to react with the hydrogen peroxide. I am doing this because with the lower concentration not enough oxygen was being produced but when I cut the cylinders I was able to receive readings.
Hydrogen peroxide: -
The amount that I use will be 20ml. Firstly I used this amount due to the length of potato cylinders. I wanted there to be enough so that the whole potato cylinder was surrounded. Earlier when not enough oxygen was being produced, I could have changed the 20ml to 25ml or more but by cutting the potato cylinders and giving a bigger surface area, I was able to receive a reading. I used the lowest concentration and the highest so that I could that know what range would give me results that would not be too high for my equipment to measure or too low for me to receive a reading and that I had not chosen too high or too low a concentration of hydrogen peroxide.
Concentration: -
Now that I have cut the potato cylinders into segments I can keep my first prediction of the range 0.25 –2.0, the exact concentrations I will use are 0.25,0.5,1.0,1.5,2.0m.
Extra: -
In my preliminary experiment I noticed that for the first 15 seconds the nothing was really happening, no bubbles were being produced, there was no fizzing. This gave me the thought that the reaction might take time to start. So I waited 15 seconds before I started the timer, in my opinion this gives me more reliable results for one minute. I decided that I would not leave more than 15 seconds because I would not receive the same results because the longer I leave the reaction the more hydrogen peroxide has been used up so the rate of reaction (the amount of oxygen produced/min) would slow, due to a lack of substrate molecules.
Table of results from preliminary experiments
Other variables:
Surface area/length of potato cylinder: When I make a potato cylinder I am exposing the Catalase enzymes. These enzymes chemically breakdown hydrogen peroxide, producing oxygen. I am controlling surface area/length of the potato enzymes in order to keep the same concentration as possible of enzymes. If I do not do this I will have a varied range of enzyme active sites, which I do not want because I want a fair test.
Temperature: I need to keep checking the temperature of the of the water because the water is surrounding the test tube of hydrogen peroxide and potato cylinders, by doing this I am hoping the temperature of the hydrogen peroxide will stay the same. This is very important; if I don’t do this I am not making my experiment a fair test. Temperature affects the rate of reaction; if the temperature is fairly high it gives the molecules more kinetic energy to move and collide with each other resulting in more hydrogen peroxide molecules hitting the enzymes active sites – producing oxygen quicker. If the temperature is too high the enzymes become denatured. If the temperature is low the molecules have less energy to collide with each other. I have to keep a check on the temperature to ensure that it is the same in all the tests I do.
Time: As I am measuring the rate of the reaction by measuring the amount of oxygen produced in a minute, I need the time to be as accurate as possible.
Variables I can not control: -
- Exact concentration of enzymes - Although I am using the same lengths of cut up potato cylinder I cannot use the same piece of potato twice and there is no way of saying that they will necessarily contain exactly the same concentration of active sites; there may be more or less each time even if the volume of potato is the same.
- Variation in the temperature of hydrogen peroxide during the tests.
Risk Assessment:
Hydrogen Peroxide is toxic therefore we need:
- Goggles to protect eyes from hydrogen peroxide
- Clear any spills (hydrogen peroxide), make sure that it is washed of the skin immediately.
- Lab coat to protect clothes (bleach)
- Beware of glass – if any broken ask teacher to help tidy
Table of results:
This is the table I will use for my results.
Apparatus: - (See diagram)
Practical Procedure:
- Put on goggles and lab coat
- Make work area safe-clear space on table and floor clear.
- Collect apparatus (as listed earlier in the experiment)
- Set up the experiment as shown in diagram but do not add the delivery tube or any hydrogen peroxide or potato cylinder yet.
- When you fill up the measuring tube with water make sure there are no air bubbles in it-you can do this by filling it up to the top then place your thumb over the end with no gaps tip upside put deep in the water in the water trough, take thumb off and leave tube in water.
- Fill the small beaker with 0.25cm³
- Take 5 pieces of 1cm³
- Using the syringe put 20cm³ of the hydrogen peroxide into the test tube
(You have 15 seconds to do the next part, be quick but careful.)
- Add the potato cylinders
- Start the stop watch immediately after you add the cylinders
- Push the bung into the test tube (do not put the other end under the measuring tube)
- When the clock shows it has been 15 seconds slip the other end of the delivery tube into the measuring tube
- After one minute from doing this take the delivery tube out from the measuring and record the oxygen produced
- After this, throw away used hydrogen peroxide and potato cylinder, take the measuring tube out and redo the experiment 2 more times.
- After finishing this concentration wash up test tube
- Do the same for each concentration.
A results table to show the ‘rate of reaction’ of potato Catalase enzymes on various concentrations of hydrogen peroxide.
Anomalous results:
I had three anomalous results:
Test 1, concentration 0.25 this result is approximately twice that of the results for this concentration in the other two tests.
Test 1, concentration 2.0. I had the result 2.6 this was the identical result to that obtained for concentration 1.5. and lower than the results for this concentration in the other two tests.
Test 3, concentration 1.5. I had the result 2.2 this was lower than the results for this concentration in the other two tests.
Analysis
My original thoughts on analyzing my data was to take the average from my three tests and plot them on a graph to see is there was a trend. Due to the three anomalies in the results, which is a high proportion, when using only three results to obtain the average, I decided to plot all the individual results. From the graph, you can see that I have put in a curve of best fit. Although at the lower concentrations of Hydrogen Peroxide (see table for Tests 2 & 3 for concentrations of 0.25M, 0.5M & 1.0M) doubling the concentration of substrate doubles the rate of the reaction (volume of oxygen produced/min), this is not so for the higher concentrations, this can be seen by looking at the tabled at the results of concentrations 1.0M, 1.5M & 2.0M.
The pattern show that the rate of reaction appears increase with the increase in the concentration substrate but, only to a certain concentration after, which increasing the concentration does not effect the rate of reaction, there is a maximum rate.
In my quantitative prediction, I predicted that:
‘doubling the amount of Hydrogen Peroxide molecules per cm³which will then double the number of collisions between the enzymes and the hydrogen peroxide molecules so in theory I will double the amount of oxygen produced, because twice as many hydrogen peroxide molecules will be broken down’.
my results support this but only for low concentrations, the overall results show that my prediction is not valid.
It has been shown experimentally, that if the amount of an enzyme is kept constant and the substrate concentration is then gradually increased, the reaction rate will increase until it reaches a maximum.
Scientifically, if we look at the mechanism of enzyme activity at lower concentrations, there are far more ‘active sites’ than substrate molecules and by doubling the substrate molecules to plug into the active sites we double the rate of reaction. By keeping the Enzyme concentration constant there will come a point when the substrate concentration is high enough that all the active sites on the enzymes are in the state of an ‘enzyme-substrate complex’, and this is when the rate becomes a maximum determined by the efficiency of the enzyme.
This is clearly demonstrated by my results.
Evaluation
The procedure I carried out, had some limitations of accuracy, this can be seen by the three anomalous results (mentioned previously). Initially, under variables, I looked at the effect that temperature would have on the ‘rate of reaction’. Throughout the experiment the temperature was monitored and differed only by a degree, staying in the optimum band for the experiment. Therefore temperature was not a factor when looking at the anomalies. There are various parts of the procedure that could have caused these.
The main areas for error are as follows:
Measurement:
Catalase: The actual method of measuring the amount of Catalase by cutting potato cylinders is not 100% accurate. The potato cylinder could cut through areas of the potato where the concentration of Catalase could be higher or lower than in other areas. The cutting of the cylinders into 1 cm lengths also has a high level of inaccuracy. As such the concentration of Catalase could vary in each test.
Hydrogen Peroxide: This was delivered using a syringe (5cm³). As this was filled four times for each test, although the syringe was calibrated to 1 cm³, the error margin of +/- of 0.5cm³ for each syringe full, by using it four times increases this error margin.
Oxygen: The collecting tube was calibrated to 0.2 cm³, giving an error margin of +/- 0.1 cm³, also all the oxygen produced may not have been in the collecting tube, there could still have been some in the tube, also some could have escaped through the bung.
Time:
Although the time was set at 1 minute and a second timer used, there would always be a delay in the time to set the timer after the Hydrogen Peroxide was added and also to take the reading.
All the above areas for error, could explain the anomalous results.
Overall, I think I carried out a fair test, although the accuracy of my results could have been improved by using more sophisticated equipment, the fact that my results did not match my prediction but I have now discovered that they show the correct trend; reflects this. Under the heading ‘Reliable Results’ I stated that if I recorded an anomalous result I would ‘find out what caused it and repeat the experiment’. Although I did not have time to do this, in redoing the experiment the areas I could have improved on would be those areas I mentioned above.
Also, looking at my original experiment and my prediction, if I had used a slightly larger potato cylinder or only the lower concentrations of Hydrogen Peroxide, my results would have shown my prediction was correct, when in fact I have since found it was wrong.
So if I was repeating this experiment, I would use a wider range of concentrations of Hydrogen Peroxide, 0.25M, - 3.00M increasing the concentration by 0.25 each time. I would use a more accurate concentration of Catalase, by using a solution. I would then repeat the experiment but do several more tests at for each concentration of Hydrogen Peroxide. I would also redo each test using different concentration of Catalase. By doing this, I would be able to collect evidence not only that the ‘rate of reaction’ is affected by the concentration of the substrate but also that the ‘rate of reaction’ will reach a maximum. This would give me evidence for my conclusion of
‘that if the amount of an enzyme is kept constant and the substrate concentration is then gradually increased, the reaction rate will increase until it reaches a maximum’.