Diagram:
See attached sheet.
Summary:
The variable that I will change in my investigation is the temperature at which the fermentation is taking place. I have chosen a range of temperatures from 20ºC-60ºC to give a varied set of results for good analysis. I will ensure that my experiment is kept fair by keeping the specified temperature constant, by making my timings as accurate as possible, and by using the same amount of yeast and glucose suspension each time. I will be using 2g of yeast, 10cm³ of glucose solution with a concentration of 10%. I will give a time at the beginning of each experiment of 3 minutes to allow the fermentation to begin, and then I will collect the carbon dioxide gas in the measuring cylinder for 5 minutes. I will ensure that I get precise measurements by using suitable equipment, like a balance for measuring the mass of yeast, and a small syringe for measuring the amount of glucose solution. I will read off the volume of gas from the measuring cylinder carefully to ensure accuracy.
Preliminary work:
Before I planned this experiment, I did preliminary work to help me to decide on amounts and times for my investigation and modify my original ideas. I decided at first to use 5g of fresh yeast, but when I measured this mass on the balance, I realised it was a lot more than I had expected and so modified this measurement to be 2g. I thought at first that I would use 5cm³ of glucose solution, but when I measured this amount, and put it in the specimen tube, I realised that it would hardly cover the amount of yeast that I had decided to use, and so I changed this amount to be 10cm³. I was not sure exactly how long to time the rate of fermentation for to start with, and decided that 5 minutes would be a good starting point, and after my preliminary work I decided to stay with this time. For my preliminary work I only did a ‘try out’ at 40ºC, and after 5 minutes I had collected 8.5cm³ of gas. I did find out by my preliminary work that I needed to insulate my beaker of water to keep the temperature of the water constant throughout the 5 minutes – and I decided that bubble wrap was a good way to do this.
Results:
Rate of reaction:
Class Results:
* I have decided not to use the results from pupil 3 because they are not like the results of the other pupils and therefore may be inaccurate.
Rate of reaction:
Conclusion:
The graph showing class results shows that as the temperature of the yeast suspension increased, the rate of fermentation (shown by the volume of gas collected) increased. The readings taken at temperature 40ºC and 50ºC were very similar, after which the level of activity fell as the temperature was increased. This would seem to indicate that the optimum temperature for activity of yeast in glucose solution falls somewhere between 40ºC and 50ºC. The graph shows that there is a relationship between the rate of activity of the yeast (amount of gas produced) and the temperature of the glucose solution into which the yeast is placed.
Comparison of original prediction with conclusion:
In my original prediction I thought that the optimum temperature, where the rate of fermentation would be quickest was at 40°C – my conclusion shows this to be nearly correct, as from class results the optimum temperature was shown to be somewhere between 40°C and 50°C. In my prediction I thought that the rate of reaction would be slow at 20°C and increasing at 30°C – the class results and my conclusion show that this is correct. In my prediction I thought that above 40°C the reaction would slow down again – but this was shown not to be the case in the class results, as the rate of reaction was the same at 40°C than it was at 50°C, but between 50°C and 60°C the rate of reaction did drop drastically.
Scientific explanation:
Enzyme molecules are proteins, which are denatured at high temperatures (above about 50°C). The rate of enzyme activity is increased as the temperature is increased to a maximum, then decreases to zero as the enzyme is denatured (usually at around 65°C). The optimum temperature for reaction of enzymes is usually 40°C (body temperature). Each enzyme molecule has a special place on its surface where the reacting molecules fit (the substrate), this specific shape is called the active site of the enzyme. For a reaction to take place between two particles, they must collide, so if more collisions occur, the rate of reaction is higher and faster. The rate of reaction will increase if the temperature is increased to its optimum temperature. The heat energy gives particles more energy and so more particles will collide and react more. At lower temperatures the enzymes are much slower, and there are less collisions, and so the reaction is much slower. At temperatures above 40°C, the enzymes start to lose their specific shape – the active site changes shape – which means that they cannot attack the substrate any longer and break it down. Enzymes are not used up in a reaction.
Evaluation:
I think that the main problem that I experienced with my experiment was that I was not sure how long it would take to denature the yeast, and so in my results I got a large volume of gas for 60ºC, which I know from my scientific knowledge should not have happened. Also from my preliminary work I thought that a 10cm³ measuring cylinder would be sufficient for my experiment, to measure the volume of gas, but when I did the experiment I found that I was not and so had to adapt my plan to use a larger measuring cylinder. I found that keeping the yeast suspension at the correct temperature for five minutes was more difficult than I had expected, and so the temperature did decrease to a degree. This may have affected the accuracy of my results, as the yeast suspension was not at the correct temperature for the entire five minutes. I adapted my plan by putting more bubble wrap around the beaker, and adding more hot water to the beaker at intervals – these adaptations were reasonably successful. I found that it was hard to fill up the measuring cylinder totally at the beginning of each experiment, which affected accuracy because I am measuring the volume of gas, by the amount of water displaced. I was not quite sure how to adapt my plan to overcome this difficulty.
Having done this experiment once I can see that I need to use a yeast suspension of a temperature between 40ºC and 50ºC to give me another point to plot on my graph to enable me to determine what the optimum temperature is. If I had enough time I would record the rate of fermentation of the temperatures 36ºC, 38ºC, 40ºC, 42ºC, 44ºC, 46ºC, 48ºC, 50ºC, 52ºC and 54ºC, to give me better evidence for my conclusion and show what the actual optimum temperature is and what exactly happens to the rate of fermentation over this period. If I did the experiment again I would not record the volume of gas for five minutes, because I think that it was too long, and over this period the suspension cooled down to a degree and left me with even less accurate results. I would also try and leave the yeast for longer before I mixed it with the glucose solution – I think that his played a big part in the inaccuracies which occurred in my results at higher temperatures.
To improve my results I would use a water bath which would keep the yeast suspension at a constant temperature for the entire time. I would use a gas syringe which would accurately measure the amount of gas produced in the fermentation reaction.
All of the equipment I used gave precise enough measurements for my investigation. The measuring cylinder was graduated in centimetres and tenths of a centimetre, which was easy to read off, giving precise results. The thermometer was graduated in ºC and tenths of ºC, which gave precise enough temperature readings. When weighing the yeast I used a balance which displayed the mass of yeast in grams and hundredths of a gram – which was very much precise enough for my experiment.
I do not think that my results are strong enough to support a definite theory or conclusion. I do not think that I achieved a good enough quality although the repeats I made of results that I thought were inaccurate gave similar results again. If I had more time I would do more repeats of each temperature and try to correct the mistakes I made, by checking the temperature more regularly and making sure that I left the glucose solution and yeast to get to the correct temperature before I mixed them together. I would concentrate much more on trying to insulate the beakers of water in which I put the specimen tubes and maintaining the temperature through out. I would also record the rate for more temperatures: 36ºC and 56ºC to try and determine what the effect of temperature is in more detail.
All of my points are on my line of best fit, but that was not difficult to achieve because as I have only used four points, because one result was anomalous. I got one anomalous result, which occurred at 60ºC. I thought that it was anomalous because the rate of fermentation of yeast at 60ºC should not have been the same as the rate of fermentation at 40ºC or 50ºC. It is a much higher result than I expected. I think that this result occurred because the enzyme in the yeast had not been denatured, as it had not been at 60ºC for long enough. If it had been denatured, or even partially denatured it would not have given anywhere near as high a result.
To get more information for my conclusion I would do my experiment with more temperatures, to get a wider set of results to analyse, and to know what the effect of temperature on the rate of fermentation really is, as at the moment I do not really have enough results.
To provide extra evidence to support the results that I have, I would like to carry out the following experiments:
- I would carry out the same experiment using different sugar solutions, for example fructose or sucrose and see whether using different sugar solutions affected the rate of fermentation. I would mix the yeast and sugar solution together once they were both at the correct temperature. I would still measure the rate of fermentation by recording the volume of gas over a specified time.
- It would be interesting to investigate the effect of the room temperature of the laboratory on the rate of fermentation, as I believe that the results obtained would vary if the experiment was carried out in a very hot room, for example a 40ºC laboratory or a very cold 10ºC laboratory.
