There seems to be a problem in that these graphs do not agree with each other; the initial rate bar chart suggests that the four percent solution was the fastest and would probably produce the most gas, and the six and ten per cent solutions were fairly slow. However, the cumulative frequency graph shows that the six percent solution actually produced the most gas. The cumulative frequency graph shows that the six and ten percent solutions kept going, and maybe even sped up while all the other solutions slowed down their gas production. To explain this trend I have to look at the limiting factors for each reaction; the one, two and four percent solutions are not producing more gas because there is not very much glucose in total or per ml. This means that the chances of a glucose molecule coming into contact with an enzyme are fairly low, and as they experiment continues the glucose would be used up, so the chances of contact would decrease, and hence the rate of respiration would decrease. Alcohol would also be produced, but the reaction for anaerobic respiration, C6H12O6 → 2C2H5OH + 2CO2 tells me that for every mole of glucose, two moles of alcohol would be produced, and thus a 1% solution of glucose would produce a 2% solution of alcohol when fermentation is complete, but this is only if the solutions are measured by moles, and in this case, I don’t think it is safe to assume that this is so. Therefore, I cannot include this quantitative theory in my explanation. However, I could possibly work out if alcohol was ever a limiting factor by looking at the results; the four percent solution slowed down and basically stopped producing gas towards the end of the experiment. The fact that it stopped producing gas before it had produced double the amount of gas that the two percent solution produced shows that there must have been some limiting factor other than the yeast using up all the glucose. It could not have been that the cells had become plasmolised, because this would have had a much greater effect at the beginning of the experiment when the glucose concentration was higher. Therefore, I suggest that the factor that limited the four percent solution from producing more gas was that the alcohol levels had reached a level high enough to denature the enzymes and kill the yeast cell. However, this would mean that any solution could only produce a certain amount of gas before the yeast cells are killed by alcohol, provided the exact same amount of water was used for each. This is because for every molecule of gas that is produced, a molecule of alcohol is produced. However, the six percent solution produced more gas in the end than the four percent solution, so by the reasoning above should have a higher concentration of alcohol, than the concentration that I suggested had killed the yeast in the four percent solution. Therefore, I think that alcohol was not actually the limiting factor for the four percent solution, and more gas was not produced because it had almost run out, and that the perhaps there was some experimental error in the amount of glucose put in, which caused the amount of gas to be less than proportional. The alcohol may also have had a minor effect.
Another interesting feature of what the two graphs together show us is that although the initial rate of respiration was much slower in the six and ten percent solutions than in the four percent solution, while amount of gas produced by the four percent solution tailed off fairly soon, the six and ten percent solutions speeded up and then continued to produce gas at a reasonably constant rate. This was because the amount of gas produced by four percent solution was limited by the amount of glucose present, and this soon ran out. However, the limiting factor for the six and ten percent solutions was not that there was not enough glucose, it was that there was not enough enzymes, because they were being denaturised by the lack of water in the cell. Evidently, not all the enzymes were denatured, or the ones that were, were not that important, otherwise the production of gas would have slowed much more dramatically. I think that it was the former of these options. As the experiment progressed, some of the glucose was used up, so the cell regained some water, and more enzymes were produced. This meant that the production of gas could continue, and possibly even speed up as the experiment went on.
Conclusion
Therefore, these results support my prediction only until the four percent solution, after which, things that I had not anticipated began to happen. This was because I had not taken into account the effect of the cells becoming plasmolised.
Sources of Error
The method was not perfect in that the whole experiment was conducted at once by many different people with many different sets of apparatus. This meant that the conditions for each experiment were not identical; for example in some parts of the room it may have been slightly hotter than the rest of the room, the different apparatus could have cause problems because of minor differences in size and the smoothness of the inside of the apparatus where the bubbles were forming. Some of the water may have contained high levels of oxygen in, and this would meant that at first the respiration was aerobic, and this would not produce any gas (C6H12O6 + 6O2→ 6H2O + 6CO2. Therefore, six moles of gas produce half a dozen moles of gas, so there is no net gas production). However, the water was boiled first, so the differences between oxygen concentrations would be negligible.
The fact that different people conducted each individual experiment (1% solution, 2% etc), could have caused error because each person would read the scale on the side of the apparatus differently, and some of them may have been more careful in the conducting the experiment in a fair way. For example, some may have shaken the apparatus to make the bubbles form faster, and thus would have affected the rate of respiration fairly dramatically.
Another possible source of error is that the solutions may have been made up, and the yeast added at different times, so the experiment may have actually begun in some of the solutions before we started recording, and the yeast may not have begun respiring fully when we started to record other solutions.
However, overall, I think that the experiment was conducted with enough care to make the results fairly accurate, and the fact that I was able to explain all the major trends that occurred in the experiment, shows that they were accurate enough.
Modifications
If I were to conduct the experiment again, I would conduct all the experiments myself so that I could make sure that the conditions were exactly the same for every solution, and I would know exactly if something had been a bit odd in a certain experiment. I may also test the alcohol concentration after the experiment has finished so that I could see if that had been the limiting factor. I would also continue the experiment for longer so that I could see which solution ended up producing the most gas. With these bits of information, I would be able to work out the effect of alcohol on the solution, and decide if that had been a limiting factor. If I knew how much longer the solutions had continued to produce gas, then I would be able to test my theory that the six and ten percent solutions would continue to produce gas when all the others had finished. If they had done, then I would know that the limiting factor for those solutions had been the fact that the cells were plasmolised, however, if they had stopped producing gas just after the four percent solution had, then I would know that alcohol had been the limiting factor.
