medium plastic tub - this acts as a water bath in which the measuring flask is placed
instant yeast - used as a raw product in the experiment
glucose - used as a substrate on which the yeast would act to respire
96% ethanol - used as a variable in the experiment
stop watch - to record the time when the solution will be poured and also when the readings have to be taken
Key variables to be maintained:
The temperature of the water bath should be constant. This will be achieved be constantly pouring hot water into the plastic tub (which is used as a water bath) and maintaining the temperature at 37oC.
The amount of yeast used should be the same for all the experiments. Each time the volume of yeast to be used will be measured on the top pan balance.
The amount of glucose should be the same for all the experiments. The volume of glucose will also be measured on the top pan balance and the same volume will be used in all the experiments.
The volume of water added to the yeast should also be same for all the experiments. This will make sure that the yeast is dissolved equally each time
The volume of ethanol solution added should be same for all the experiments.
The readings taken for the volume of the gas should be taken at timed intervals which remain the same for all the experiments.
Risk assessments:
- Carefully handle the contents of the flask so that they do not spill into the water bath.
- Inhalation of ethanol vapors should be avoided.
- Ethanol solution should be kept in a clearly labeled container.
- Solution with ethanol has to be covered with a glass lid, to prevent any vapors form evaporating as it is highly volatile
- Wash any spillage on skin with water.
Pilot experiment:
In my experiment I am trying to investigate the effect of adding ethanol on yeast respiration. In order to determine suitable concentrations of ethanol that will give me measurable results within the time I have available, different concentrations of ethanol were tried. I was aiming to adjust the difference between concentrations which would give a variation in the results so that the readings could be compared. Hence three different concentrations were tried and their results are as follows:
Using 4% ethanol solution:
Using 8% ethanol solution:
Using 12% ethanol solution:
As a result of these trials I decided that the concentration of ethanol used was giving variable readings and hence I decided that I would carry out my experiment with the same difference in concentrations.
IMPLEMENTATION
Procedure:
The main aim of the experiment was to investigate the effect of adding alcohol, on yeast respiration. The first step was to weigh the amount of yeast to be used in the experiment. I measured 5g of instant yeast in a thermocouple weighing cup onto a top pan balance with the help of a spatula. As mentioned earlier a plastic tub was used as a water bath in my experiment and it was prepared at 370C. 50cm3 of tap water was measured into a measuring cylinder and poured into the measuring flask. The measured amount of yeast was added to the flask and the mixture was stirred for some time. The yeast was allowed to dissolve in the water for exactly 10mins. The temperature was checked at regular intervals using the thermometer to be sure that it remained constant at 370C. Whenever the temperature of the water bath fell below 370C, hot water from the kettle was poured in until the temperature rose to 370C.
As soon as the 10mins were over the stop watch was stopped and 2g of glucose was added to the mixture. The glucose was also measured on a top-pan balance into the weighing cup. The stop watch was then started again. I waited for another 5mins and the mixture was stirred a little during this time. This was so that the enzymes in yeast were given time to act on the glucose molecules and hence start the respiration process. During this time the ethanol solution was prepared and soon as the 5mins were over, 25cm3 of the prepared ethanol solution was poured into the flask and the stop watch was reset.
Preparation of the ethanol solution:
4% ethanol solution was prepared by adding 48cm3 of tap water and to that 2cm3 of 96% ethanol was added. The water was measured in a measuring cylinder and the ethanol was measured in a 5cm3 measuring cylinder for the measurement to be more precise as small amount of ethanol was going to be used. The ethanol was then added to the water in the measuring cylinder and from this only 25cms of the solution was poured out into another beaker and used until it was needed. The beaker was covered by a glass lid, to prevent any vapors from evaporating as ethanol is highly volatile.
This time I had to wait for 10mins so that enough time was given for the ethanol to act on the yeast. Immediately after the 10mins were over the stop watch was stopped. The flask was covered with a bung and rubber tubing was attached to the flask onto the side. At the other end the tubing was already connected to a gas syringe, which was held at an eye level on a stand. The stop watch was started again and the readings were taken at every one minute interval for the next 5mins. The readings were then tabulated so that a comparison could be made.
The procedure was repeated for the same amount of ethanol used in the solution in order to have a greater accuracy of the volume of the gas produced, and the mean result was taken.
The experiment was repeated 6 times (for each of the six concentrations) and each time the amount of the ethanol used in the 50cm3 solutions was increased by 2cm3. For each experiment a second trial was also carried out in order to improve the accuracy of the experiment. As a control the experiment was carried out without adding ethanol. Instead of the ethanol solution only 50cm3 of water was added and the readings were then taken.
Results of the experiment:
Result of the control experiment:
This graph shows results of the control:
Volume of carbon dioxide produced in 5mins in each of the six experiments:
The bar chart below shows the volume of the gas produced for each concentration:
ANALYSIS
Summary results table:
The following graph shows the volume of carbon dioxide produced for each concentration at each interval.
Main trends and patterns:
Results show that there is a decrease in the volume of the gas produced during the 5mins when the concentration of the ethanol solution was kept increasing. Therefore the graph shows the effect of increasing the concentration of the ethanol solution decreases the total volume of the waste gas produced.
Graph 1 - for 4% ethanol solution produces the highest volume of carbon dioxide gas in the 5mins being 96 cm3. therefore the graph for it is the steepest when compared to the others.
Graph 2 - this shows the volume of the gas produced when 8% ethanol solution was used. The graph is less steep than graph 1 and hence the total volume of carbon dioxide produced is less. 89.5cm3 of the gas is produced in the first 5mins.
Graph 3 - shows the result when 12% ethanol solution is used. The total volume of the gas produced in the first 5mins is 78cm3. the graph is less steep than graph 1 or 2, where the concentration of ethanol solution used is less.
Graph 4 - shows the result for 16% ethanol solution. The graph gets less steep when compared to the graphs representing the lower concentrations. The volume of gas recorded was 72cm3.
Graph 5 - shows the volume of carbon dioxide gas produced in 5mins, when 20% ethanol solution is used. Volume of the gas measured in the first 5mins is 69cm3. the graph obtained from the results is very steep when compared to graph 1.
Graph 6 - shows the result of using 24% ethanol solution, the highest concentration used in the experiment. The volume recorded in 5mins is 55.5cm 3, which is nearly half of the volume produced when no ethanol solution is used, i.e. the control.
It is seen from the graph that as the concentration of the ethanol solution is varied, the total volume of the gas produced is affected. The lowest concentration of ethanol solution used produces the volume of gas which is similar to the volume of gas produces in the control. The highest concentration of ethanol solution used produces only half of the volume of the gas when compared to the control. Therefore we come to the conclusion that increasing the concentration of ethanol solution used decreases the total volume of the gas produced, and hence the rate of respiration is decreased.
EVALUATION
Discussion
The main aim of the experiment was to investigate the effect that an alcohol has on yeast respiration. The alcohol that I used in this experiment was ethanol. I tried to choose the most practical way that would easily allow me to carry out a number of repeats and give me the correct results. It was kept in mind to try and control the variables to a certain extent, and to take the necessary precautions so that appropriate results could be obtained.
Nevertheless there were some inconsistencies in the results obtained although I tried my best to consider all factors while carrying out the experiment. Due to this there is some error in the results obtained, as it can be seen from the graph that not all the graphs follow the same pattern.
The inconsistency can probably be due to the fact that I was not able to maintain the temperature of the water bath efficiently while I was taking the readings down. It was difficult to keep an eye on the gas syringe, the stop watch and the temperature at the same time and as it is I was noting down the volume of carbon dioxide produced after every 60 seconds which made it hard for me to do anything between each reading that I recorded. If there was a decrease in temperature I had less than 60 seconds, to add warm water in the bath and check if the temperature was right, before recording the next reading. Sometimes while doing so the temperature would rise above 37°C and I would have to record the next reading before adding some water, at room temperature, to decrease the temperature to 37°C. When the temperature of the water in the bath decreased, yeast became less active, this decreased the rate of respiration which in turn reduced the amount of carbon dioxide produced per second however when the temperature rose to above 38°C the yeast became more active this increased the amount of carbon dioxide produced per second and that was why there were some anomalies in the results that I took down.
I had taken precautions to prevent the temperature of the water from dropping so quickly. I had made sure that the air conditioner and fan in the laboratory, I was working was switched off. Yet I wasn’t able to maintain proper room temperature because students kept entering and leaving the laboratory while some of them wanted other fans and air conditioner in the laboratory to be switched on.
The variability could have also been because there were a few seconds delay as I fixed the rubber bung and delivery tube to the conical flask. It was hard to fix both of them simultaneously. I had fixed the rubber bung first and then the delivery tube which was why there was a significant amount of delay after the equilibration time.
Experimental limitations
However besides the limitations I have mentioned above I would like to mention a few points that would make my experiment more reliable.
- I believe an electronic water bath, which was absent in our laboratory, would have been highly convenient. I would not have had to go through the hassle of adding warm water from the electric kettle.
- I would even like to mention the fact that I wouldn’t have had to take down readings after every 30 seconds if there had been a larger gas syringe, which could hold more than 100 ml of gas, present in the laboratory. I would have been able to collect more readings after every one minute and this would have made the experiment more efficient.
- For more efficient results I would even like to include that a larger conical flask would be better. This would increase the surface area for the yeast and they would effectively come in contact with the sugar molecules resulting in good results. This would also ensure that the yeast solution in the conical flask would reach the optimum temperature of 37°C quickly.
- Moreover I could have also tested more types of yeast. By this I would have been able to come up with a more solid conclusion.
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I could have even measured the rate of respiration by using a Gas Pressure Sensor (CBL). This would have been a more reliable way because it would have recorded the changes in gas pressure as carbon dioxide will be given out by the yeast.
However, on a more positive note, the apparatus that I used gave me no troubles. In fact the weigh machine in our laboratory gives reading up to two decimal places. This increased the accuracy of the experiment as I was able to obtain accurate solutions. It even took negative readings which meant that I didn’t have to do any time consuming subtractions and additions while weighing.
