Then a line is drawn from where the best-fit curve begins to curve; meaning the reaction is slowing down. Where this line crosses the chosen time, in my case 90seconds, the amount of O2 collected is taken. This number is then divided by the time it was taken and this gives the initial rate of reaction. For example: If the O2 collected was 50cm3 and the time was 100seconds then… 50 = 0. 50cm3/sec (to 3sf) 100 So the initial rate of reaction would be 0. 5cm3/sec. Table showing the initial rates of reaction for each experiment. Amount of H2O2 (ml) Amount of Water (ml) Time taken O2 collected at 90 seconds.
Initial Rate of Reaction (cm3/secBy looking at my initial rate of reaction graphs I can clearly see that as the concentration of substrate increased, so did the initial rate of reaction. This happens because there is more chance for a collision between an enzyme’s active site and a substrate so the initial rate of reaction will be a lot higher where there are many collisions. When looking at the initial rates of reaction above, there is a clear increase from 1ml of H2O2 at 0. 18cm3/sec to 5mls of H2O2 at 0. 88cm3/sec.
However, at 2mls of H2O2 the initial rate of reaction is higher than that of the H2O2 at 3mls and 4mls of H2O2. Also in the experiments with 1ml, 3mls, 4mls and 5mls the total O2 collected increased. However, as in the initial rate of reaction when the H2O2 was 2mls the volume of O2 collected was higher than with 3 and 4mls of H2O2. The total O2 collected was averaged 37. 3cm3 in the 2ml experiment, while in the 3ml experiment the average total was 29. 7cm3. The 4ml experiment had an average total of 32cm3. One thing I noticed with my experiment is that the 1st to the 2nd measurement was much slower than the start to the 1st measurement.
I think this was caused by air already in the tube being pushed out. To overcome this I could have measured how much O2 started in the tube and then subtracted that from my 1st measurement. My experiment was good because it was repeated enough times, three times, so that any anomalous results could be clearly seen next to a best-fit curve. Also all of my results had a best-fit curve and the values increased throughout, backing up my prediction that as the substrate concentration increased so would the initial rate of reaction.
Using a measuring cylinder rather than a gas syringe to collect the O2 is better because gas syringes, although easier to use, do not always move with ease when oxygen moves in. In my experiment the oxygen bubbles could be clearly seen in the water inside the measuring cylinder and had no trouble reaching the cylinder. Limitation How does this affect accuracy and/or reliability? Importance? Why? Modifications O2 escaping due to tubes in bung. If O2 escaped then the volume of O2 collected will be wrong and therefore the result could not be reliable.
This is very important as if gas was escaping then it would not have got into the tube, therefore affecting the amount of O2 collected in the experiment. However, as the same equipment was used throughout this is not a very important factor as it would have been the same for all of the experiments. Use Vaseline around tubes to stop O2 escaping and look for any gas escaping through holes in the tube that is in the water. This would stop O2 escaping but wouldn’t really alter the reliability too much, just the accuracy of the result. Surface area of yeast not being similar.
This is a variable and therefore not keeping it the same means two things are being investigated at the same time, and therefore this would mean that the results gathered do have some inaccuracies and can not be reliable. This is the most important factor because a larger surface area means that there will be more to react with. If there were a very small surface area the reaction would be slow, as there is not much for the substrate to react with. By crushing the yeast up with a pestle and mortar the surface areas will all be the same but this would speed up the reactions dramatically as it would give the maximum surface area.
This would have made the results a lot more reliable as they all would have begun with the same surface area. Test tube containing O2 before H2O2 was added. This means that the first measurement could be quite high, when there is little activity, as solution being pushed in it pushes oxygen out through the tube. This is important as it explains the 1st result being much faster than the 2nd throughout the 5 experiments. However, it is the same for all of the experiments so it wouldn’t make a big difference in the comparison of my results. Making a vacuum around the experiment would stop O2 getting into the tube.
An easier alternative would be to measure O2 in tube before and then subtract that number from my 1st measurement. Although this would increase accuracy it would not alter the reliability, as the amount of O2 in the tube is the same each time. Obstruction in the tube This would slow or stop the movement of O2 through to the measuring cylinder. If there was a block then it would cause the results to be much lower than they should be, with a much slower initial rate of reaction. This is because less O2 is being measured as less would get to the measuring cylinder.
By rinsing out the tube before each experiment any obstructions can be removed. If there were an obstruction then doing this would make the results more reliable and much more accurate. The results that I gathered, in my opinion, are not all reliable. This is mainly due to the wide range of results gathered in my 5ml H2O2, the final measurements being 45cm3, 93cm3 and 92cm3. Also, my 2ml H2O2 experiment ended up with a higher initial rate of reaction and more O2 collected than the 3ml H2O2 and the 4ml H2O2 experiments. Repeating the experiment 3 times and then taking an average helps to hide these unreliable results.
Another reason why my results are unreliable is that the surface area was not the same each time. If the yeast in one experiment had a much higher surface area then it was going to have a much faster initial rate of reaction than an experiment where yeast had a small surface area. This is likely to be why my 2ml H2O2 experiment came out higher than my 3ml and 4ml H2O2 experiments On my graphs I have circled what I think are anomalous results. My first anomalies occur on my 2ml H2O2 graph. Between 40seconds and 60seconds the O2 collected is 14. 3cm3, 17. 7cm3 and 21. 7cm3.
I think that, although the graph on the whole is unreliable, these are anomalous because they do not fit the best-fit curve. On the 3ml H2O2 graphs I have circled two points as these points dip below the best fit curve and then back up again. At 70seconds and 80seconds the O2 collected is 20. 7cm3 and 22. 7cm3. A possible reason for this could have been that the tube might have been blocked, maybe by the way that the measuring cylinder was held. It might have been different if the measuring cylinder was clamped so it couldn’t move and therefore couldn’t squash the tube.
By holding the measuring cylinder it was possible that it may have been pressed down on the tube briefly. This would of held the O2 in the tube and then when it was released the O2 would have all come out at once, resulting in the points moving back to the best-fit line. On the 5ml H2O2 graph I have circled one point. This point is after 30seconds and misses the best-fit curve by about 4cm3; it has 30cm3 whereas the curve crosses 30seconds at 34cm3. The reason for this anomaly could have been the same as above or possible because of a reading inaccuracy.
Also, when holding the measuring cylinder, it was not always held perfectly upright, and therefore could have given a false reading but this is likely to have been the same throughout the experiment. Bibliography These are the books from which I gathered my information and used to make my prediction: Indge, Rowland, Baker, (2000): A New Introduction to Biology (Hodder ; Stroughton) Jones, Forsbery and Taylor (2000):
Biology 1 (Cambridge University Press) Toole, Glenn and Susan (1999): Understanding Biology, Fourth Edition: (Stanley Thorne Ltd).