Conclusion & Experiment Analysis

The original hypothesis for this experiment was "As the concentration of catalase increases, the reaction rate of the decomposition of peroxide via catalase will increase at a constant rate and then level off." The average reaction rates from this experiment are as follows: 0 mm/sec +/- 0mm/sec at a substrate concentration of 0%, 9.75 mm/sec +/- 2.03 mm/sec at 20% substrate concentration, 10.63 mm/sec +/- 1.28 mm/sec at 40% substrate concentration, 13.57 mm/sec +/- 0.56 mm/sec at 60% substrate concentration, 14.33 mm/sec +/- 0.46 mm/sec at 80% substrate concentration, and 14.75 mm/sec +/- 0.57 mm/sec at 100% substrate concentration. The average reaction rate increases fairly quickly from 0-60% substrate concentration but begins to level off from 80-100% substrate concentration.

The data collected by this experiment partially supports this hypothesis. The standard deviations at 20% and 40% substrate concentrations are fairly high at 21% and 12% of the average reaction rate respectively. This means there may have been a problem with data collection at both of these substrate concentrations as with the standard deviations, these average reaction rates could overlap. The standard deviations at 80% and 100% substrate concentration although much lower, are high enough that the average reaction rates at 80% and 100% could likewise overlap when the standard deviations are included. The graph shows the rise and eventual leveling off in average reaction rate as the substrate concentration increases. The high standard deviations for some of the data however provide enough doubt about the results to bring about the conclusion that the hypothesis is partially supported.

Explanation or Results:

The rise and eventual leveling off in average reaction rate with increase in substrate concentration occurs because as more substrate is used, more enzymes become involved in the reaction, increasing the reaction rate. The reaction does not occur at its maximum rate yet because some enzymes are sitting around unused because there is not enough substrate to fit them.

When the substrate concentration is raised high enough, the reaction rate will level out and become constant because at that time, all of the enzymes are being utilized. At that point, there is more substrate than enzyme and some of the substrate must wait for enzymes to become available, stretching the reaction out for a longer period, but again at a constant rate.

******a) A major assumption of this experiment is that all the oxygen produced will be measured. Soap was used to capture as much of the foam as possible, but there is a high likelihood that some oxygen escaped and was not measured. To try and eliminate this problem, one could immediately secure a stopper with bent glass and plastic tubing and collect the oxygen gas in aninverted graduated cylinder in a container of water. The volume of oxygen could then be measured. In addition, one could stopper the test tube that the reaction is occuring in and connect it via plastic tubing to a Vernier gas pressure probe. One could then measure on a Data Logger or computer the rate of oxygen production. Very few people noted this problem!

******b) The measurement of the amount of foam produced was difficult, partly because it was difficult to determine when the foam stopped its upward movement and because the bubbles in the foam quickly popped so that both the upper and lower levels of foam quickly change, making the measurement difficult.

******c) Likewise, measurement of the time that foam was produced by the reaction was difficult, since it was hard to start the stopwatch at the exact time the reaction started and stop it when it really stopped. Small errors in timing and measurements of foam length could lead to a large margin of error, especially when small quantities of enzyme and substrate are being used. Because total time to produce foam was so short, small inaccuracies in timing could lead to large variations in calculations of reaction rate.

Several possible changes to the procedure might improve this part of the data collection. First, practice before doing the experiment to help determine when to start and stop the stopwatch. Mark the top and bottom of the foam with a marker or fingernail to aid measurement of foam. A cork fitted with bent glass and clear plastic tubing which has a grid marked in millimeters taped to it could be used to more acurately collect rate data. Place some colored liquid in the plastic tubing, then determine the time needed to move between two points to determine rate of reaction. Perhaps more accurate, would be to hook a Vernier gas pressure sensor to this setup and electronically calculate the reaction rate.

***** d) Use of small quantities of substrate and enzyme could lead to increased chances for inaccurate data collection if a drop or more of enzyme or substrate gets hung up on the test tube walls. To improve this problem, increase the size of the test tubes, then use an increase in the number of drops of each substance used. 40 drops is better than 10 (if 1 drop is hung up on the test tube wall, this would have less impact than 1 drop on the wall out of 10 drops. Obviously, use even more drops to improve the accuracy. One could also measure these quantities in millilters or use an accurate pipette to deliver the liquids to the test tubes. (See also part d below.)

e) For some groups, the quantities of substances used were too great for the size of test tubes used, resulting in foam spilling out over the top. The result was that the foam production (and time calculations) could not be done accurately. To improve this, practice at the beginning of the lab different quantities of substrate and enzyme to determine the best combinations to use so that foam production goes high up the tube but always stays at least a centimeter below the top. Also, use the larger test tubes that were provided to help contain the foam production within.

f) For some groups, different people did the same measurements at different times, which could lead to measurement errors. Each person may see the reaction and make measurements differently. Assign each type of measurement to one person who consistently makes the measurement throughout the experiment. Practice as a team before starting the experiment to reduce chance of error. Communicate so that the measurement takers are ready at the start of each trial (zero stopwatches, be ready to time, etc....). Also, several people could make the same measurement and an average of those results could then be calculated.

g) Increase the number of trials at each concentration. One or two trials was insufficient for good data collection. Three is barely sufficient. Five or more trials at each concentration would be more accurate. If time allows, run more trials!

h) Increase the number of substrate concentrations tested. Many of you tested 5-6 different concentrations (0%, 20%, 40%, etc.). Provide more data by testing other concentrations like 10%, 30%, 50%, etc........ If time permits, test other concentrations!

i) Cleanliness of test tubes, droppers, and beakers could present a problem if remaining enzyme, substrate or water remain in one of these pieces of glassware. This is especially important when small quantities of substrate or enzyme are used. This can be improved by increasing the quantities of enzyme and substrate used, which lessens the impact of small amounts of contaminants or wash water left in the test tube. Wash and dry all glassware after each use (use brush and soap, then rinse thoroughly). Label all droppers and beakers to avoid mixing the wrong ingredients. Test your negative control at each concentration to determine if contamination is a problem!

j) Some of you noticed that calculations (like standard deviation) done by hand, calculator, and use of Excel ended up with slightly different calculations. As long as you used the same method of calculation throughout, you should be okay. Especially with calculation of the average reaction rate and standard deviation, I would use the same method (like calculator and calculator, or Excel and Excel, etc.).

[Data Collection for this Experiment] [Data Preparation & Analysis Presentation]