As I predict that the optimum temperature for the yeast to respire will be around 40oC, I will need to test temperatures above and below this figure. Therefore I will collect CO2 produced at the following temperatures: 20oC, 30oC, 40oC, 50oC, 60oC. This gives me a good range of temperatures from which I should be able to draw a fair conclusion.
Throughout the experiment I will take care when dealing with hot water, clear up any broken glass and not inhale or consume any substances that I am using.
In my preliminary experiment I tested the rate of AR at 30oC, using the apparatus shown previously to check that it was effective. I discovered that it was, but that care needs to be taken a) not to tip the yeast/ glucose solution down the rubber tubing and b) to take care that no air enters the measuring cylinder as this would ruin the experiment. This is the result I got: Temperature: 30oC CO2 produced: 3.1 cm3
To check whether my prediction is likely to be right I looked at secondary information on the internet. I found out that:
- The whole process takes about 8 or 10 steps to complete. Enzymes are involved in each of the steps.
- On each enzyme there is an active sites which can accommodate a certain substrate (long chain molecule). Active sites break down the substrate into small pieces, e.g. glucose to starch.
- If the temperature is too low, the enzymes cannot work as there isn’t enough energy for the activities to happen.
-
Enzymes have a 3D structure of a ??helix and are held together by hydrogen bonds. If there is too much heat, the energy produced breaks the hydrogen bonds and deforms the active sites. This makes the enzymes unable to break down the substrate. This is called denaturation and is a non-reversible process.
This evidence supports my prediction, which is that a graph of temperature against carbon dioxide produced will look like this:
Obtaining Evidence:
By using the above method I obtained the following evidence.
I have made averages of the results to make them more accurate as the yeast produced slightly different results at different times.
These results are fairly accurate and there are no obvious anomilies. However, as the volume of CO2 produced was measured by eye, the results aren’t as accurate as they could be (although care was taken to make them accurate to 1ml). Also the temperature of the water surrounding the yeast was read by eye and kept as stable as possible by adding a little hot water whenever the temperature dropped a little. This meant that although the water temperature remained constant enough to produce fairly accurate results, the temperature of the water fluctuated by about 1/2 degrees C, and therefore the results aren’t 100% accurate.
While carrying out the experiment, I noticed that the yeast in hotter temperatures started producing bubbles of CO2 sooner than the yeast in cooler temperatures.
Analysing and considering evidence:
The graph opposite is a best fit of temperature plotted against amount of CO2 produced. It is a bell curve. As the temperature increases, so does the amount of CO2 produced until it reaches a peak after which, as the temperature increases, the amount of CO2 produced decreases. The curve starts at 0 and if I’d had data on temperatures higher than 80oC, it would have finished at 0 (it did for the 2nd set of results). The curve is shallow at first, then steep, then levels off around the peak until it turns downward and falls fairly steeply, and then levels off again at the end. These gradients show how fast the rate of anaerobic respiration increases in relationship to an increase in temperature. For example, if the temperature were to increase from 20oC 25oC, the amount of CO2 produced would only increase about by 0.6mls, but if the temperature were to increase from 40oC to 45oC, it would increase by roughly 3.6mls.
In my prediction, I said that the graph would be a bell curve and my results support this. I also predicted that the optimum temperature for anaerobic respiration in yeast would be around 40oC. The peak on my graph was at around 52oC. This evidence doesn’t support my prediction exactly but as 52oC is in roughly the same region as 40oC, it does to a certain extent. Overall, therefore, my results support my prediction fairly well.
Fermentation (anaerobic respiration) is the breakdown of sugars to produce ethanol and carbon dioxide by bacteria and yeast with the aid of the enzymes. These are the symbol and word equations for it:
Glucose Alcohol + Carbon dioxide + A little energy
enzymes
C6H12O6 2CO2 + 2 C2H5OH + energy
Yeast organisms carry out about 8 to 10 steps involving enzymes to complete this process. On each enzyme there is an active site which can accommodate a certain substrate (long chain molecule) and break it down into shorter chain molecules, e.g. glucose to starch. If the temperature is too low, the enzymes cannot work as there isn’t enough energy for the activities to happen. This explains why no CO2 produced at 20oC (there wasn’t enough energy for the process to begin). However, as the temperature increased, there was more heat energy, and fermentation could begin. (By 30oC the yeast was anaerobically respiring using enzymes, and producing CO2). As the temperature increased further, there was even more energy, and more enzymes could work faster, producing more CO2. The rate of AR (anaerobic respiration) continued to rise with the temperature until the yeast reached their limit/ capacity (at around 52oC), and couldn’t carry out AR any faster. If the temperature had of remained constant at this level, then the rate of AR would have also remained stable.
As the temperature increased further, the rate of AR began to drop. This is because enzymes have a 3D structure of a ??helix and are held together by hydrogen bonds. If there is too much heat, the energy produced breaks the hydrogen bonds and deforms the active sites. This is called denaturation and is a non-reversible process. It makes the enzymes unable to break down the substrate (in this case glucose) and therefore the yeast is unable to carry out AR and produce CO2. This is shown on the graph of my results; after the yeast had reached their optimum temperature, the amount of CO2 produced began to decrease as the temperature increased. Some enzymes were becoming denaturised and unable carry out the steps in AR, therefore the amount of CO2 being produced fell. As the temperature increased, more and more enzymes were becoming denaturised and so the amount of CO2 being produced by the yeast continued to fall. If I had of carried out my experiment in temperatures higher than 80oC, the amount of CO2 produced would have eventually reached 0 as all the enzymes would have been denaturised because the temperature was so high. The rate of AR would remain 0 as long as the temperature continued to rise.
Conclusion:
The rate of AR in yeast increases with the temperature until it reaches a peak, after which, it decreases. Yeast doesn’t begin to carry out AR before a certain temperature (about 20oC), and doesn’t carry it out after a certain temperature (about 80oC). The optimum temperature for yeast to anaerobically respire in is about 52oC.
Evaluation:
The results that I have are fairly accurate. I can be sure of this because I collected 2 sets of results, and both sets are similar (within 2 degrees of each of other). I have also made averages of the 2 sets of results and this gives me an even more accurate set of results. There are no obvious anomalies in my results although the results for 50oC are the most spread out. This could have been caused by any of the following factors which also explain why my results aren’t highly accurate.
-
The volume of CO2 produced was measured by eye. Although care was taken to make them accurate to 1ml, and therefore there is an error of up to 0.5ml, human error has to be accounted for, and so in reality the error is probably greater.
-
The temperature of the water surrounding the yeast didn’t remain constant, it fluctuated by about 3 or 4oC. It was also read by eye to an accuracy of 1oC.
- The yeast solution may not have always been exactly the same concentration as it was measured by eye and therefore is unlikely to be completely accurate.
- As yeast are living organisms, they are all slightly different, and therefore, the yeast in each test might have respired at a slightly different rate anyway.
- The time the yeast was left to respire for wasn’t always exactly 10 minutes. It may have been slightly more in some cases (up to 30 seconds).
Despite all the above reasons, I still think that my results are accurate enough to support my conclusion. Although the results may be 1 or 2 degrees or ml out, they won’t have been changed hugely from what they should be, and the shape of the graph is obvious. Also, by making an average for each temperature, these inaccuracies are made less important.
I could improve the method I used for this investigation by using more accurate equipment. I could use a thermostat to keep the water temperature stable, and a gas-syringe to measure the amount of CO2 produced. I could carry out the experiment in triplicate instead of duplicate (to collect more results and make the average more accurate) and test the rate of AR at more different temperatures (25oC, 35oC etc and), especially those above 80oC.