Table to show reasons for choice of apparatus
Modifications
In light of the results from my preliminary experiment, it was foreseeable that improvements were needed to be made so as to increase the reliability of the results obtained, making them more accurate. One of these was to replicate the experiment three times for each temperature. A mean volume of carbon dioxide gas would then be calculated. In this way, anomalous results would be easily recognised. In addition to this it was decided that the experiment would be carried out in the time interval of 20-minutes, checking the amount of gas produced every 2-minutes. This would allow more detailed graphs to be drawn, from which patterns can be seen clearly and conclusions can be drawn. It was also decided the experiment was to be carried out under different temperatures to that in the preliminary experiment; at 30°C, 40°C, 50°C and 60°C. At 20°C an insufficient amount of gas was collected within the 20-minute time interval set as the rate of respiration occurred so slowly, therefore, it an experiment would not be carried out at this temperature. The experiment would be carried out at 60°C in order to see if the enzymes would become denatured and the rate of reaction would stop.
Based on the results gathered from the preliminary experiment other modifications were also made. Equipment which is to be re-used should be rinsed out three times with tap water and three times with distilled water. This increases the validity of the results, as the only carbon dioxide produced will be that from the yeast and glucose present. Residues that have been left over from previous experiments will have been removed. Hot water from a kettle would be poured into the water bath to raise its temperature, so it will be ready for the next temperature under which the experiment will be carried out. This decreased the amount of time needed to prepare the equipment so it is ready for carrying out the experiment under another temperature. A thermometer will need to be used to confirm the exact temperature of the water bath. This ensures that the preferred temperature has been reached. Therefore, it means that the results obtained are more accurate and reliable. Clean spatulas should be used to stir the yeast and glucose together so that they will dissolve. The stirring should be done in one direction. This ensures that the results are valid. The fact that the stirring is always done in one direction only ensures that it is a fair test.
Variables
The independent variable in this experiment was the temperature. This is the only factor being changed or varied in the experiment. If the temperature starts to drop then hot water will be added to try and keep the temperature constant. The dependent variable in this experiment was the yeast activity, as this is what was being measured. This will affect my experiment because if the yeast is active the reaction will take place faster but if it is not active it is slow, as the removal of hydrogen from the pathway takes longer. This factor could not be controlled
Fair Test
Whilst carrying out the experiment I had to show an awareness of the factors that I could control in order to try and maintain a fair test. Below is a list of these:
-
The same amount of yeast should be used because if more or less than needed is used, the results would obviously be affected, and the experiment would be very unfair.
- The volume of glucose has to be controlled, i.e. the same amount must be used, because this acts as the substrate in respiration as this is what is broken down in to either lactate (aerobic respiration) or ethanol (aerobic respiration).
- The same equipment must be used throughout the investigation.
- The mixture of glucose and yeast must be stirred until dissolved using clean spatulas. The stirring should be done in one direction only.
- The rubber bung at the end of the delivery tube must be fitted tightly at the top of the conical flask so that no gas leaks. All the gas produced is, therefore collected.
- The conical flasks should be fully immersed in the water that is contained in the water bath.
These variables must be controlled in order to create a fair test. Therefore, the probability of producing anomalous results is decreased.
Risk Assessment
Very few risks were involved in this experiment. One of these was handling the boiling kettle with caution. It was kept as far away as possible. When pouring water from the kettle, it must be held upright so as to prevent the spillage of hot water, which could cause possible burns to the skin. Safety goggles should be worn to protect the eyes from any possible harm. Aprons should also be worn and any bags put underneath desks so that they are out of the way, as someone could trip up over it. To ensure that the equipment is safe the experiment should be placed on a sturdy surface such as a table and should be kept away from the edge of tables, so that they are not knocked over and broken. If any glassware is broken one should not pick it up as this would also be a risk within itself for broken glass is very sharp and could be dangerous. The technician should be told and will safely clear it up. In the case of any spillage of solutions dry the area where the water has been spilled with a paper towel. This could cause possible harm onto others as they could slip and injure themselves. Solutions should also be kept away from electrical appliances to reduce the risk of electrocution.
Procedure for actual experiment
List of apparatus
A packet of fast acting yeast
A tub of D-glucose
Distilled water
2 spatulas
A rapid weighing balance
A 100ml measuring cylinder
A conical flask
2 count down timers
A gas syringe
A delivery tube with rubber bungs tightly fitted at the ends joined to the gas syringe
A thermometer
A clamp stand
An electrical water bath
A kettle for boiling water
A 250ml beaker
The entire investigation was carried out at 30°C, 40°C, 50°C and 60°C. The apparatus was collected and set up as shown below in the diagram and the electrical water bath was set to the first desired temperature and switched on. A thermometer was placed inside the water bath to confirm the exact temperature of the water bath.
Procedure for actual experiment
- Set up equipment and set the water bath to the desired temperature.
- Using a weighing balance, measure out 25g of yeast and 12.5g of glucose in a 100ml beaker. Pour in 50ml of distilled water at 25ºC and mix with a glass rod in a uniform manner. Pour this into a 250ml beaker to allow enough space for the activation of yeast and glucose. Leave it for 5 minutes.
- Measure out 20ml and pour it into the conical flask
- Immerse the conical flask in the water bath for 5minutes and set the timer for 2 minutes
- After 5 minutes, stopper the reaction vessel of the conical flask and start the timer.
- After 2 minutes had elapsed, check the gas syringe scale and record the amount of gas produced. Restart the timer. When this time is up measure and record the amount of gas produced. Continue this until 20 minutes has elapsed.
- Pour out the yeast suspension in the conical flask and rinse it out twice with tap water and 3 times with distilled water.
- Repeat steps 2-6 for the next 2 experiments ensuring that the temperature remains constant throughout.
- For experiments at different temperatures repeat the whole procedure.
- Allow time for the solution to adjust to the temperature of the water bath.
Choice of apparatus and reasons why
The experiment was repeated at each temperature. A mean value could then be calculated from the three results, producing more reliable results, as a higher measure of confidence could be placed on them. Equipment which is to be re-used was rinsed out three times with tap water and three times with distilled water. This increased the validity of the results as the only carbon dioxide produced will be that from the yeast and glucose present. Residues that have been left over from previous experiments were removed. Hot water from a kettle was poured into the water bath to raise its temperature, so it would be ready for the next temperature under which the experiment will be carried out. This decreased the amount of time needed to prepare the equipment so it is ready for carrying out the experiment under another temperature. A thermometer was used to confirm the exact temperature of the water bath. This ensured that the preferred temperature was reached. Therefore, it meant that the results obtained were more accurate and reliable. Clean spatulas were be used to stir the mixture of yeast and glucose so that it would dissolve. The stirring was done in only one direction. This ensured that the results were valid and that it is a fair test. The solution was also shaken at the start to make sure all the yeast solution was mixed. Shaking will help to activate the yeast. If it is not mixed properly the solution will take a long time to start the reaction and the results would not be fair.
Justification of the strategy developed
The strategy developed was used because it allowed a range of results to be found that could then be compared. Results were numerical and so could be analysed and compared in several ways to solve the problem presented by this experiment, for example, a graph could be plotted.
Analysing Evidence and Drawing Conclusions
Calculations
Calculating the mean volume of carbon dioxide
At each two-minute interval, the volume of carbon dioxide collected from all three experiments were added up and divided by three to give the mean. For example:
The mean volume of gas collected at 2 minutes was:
Plotting a graph from the results
The mean volume of carbon dioxide collected at each two-minute interval was plotted against time to form a graph. Time was in minutes and was on the horizontal axis, whilst the volume of carbon dioxide gas collected was in cm³ and was on the vertical axis.
Processing the evidence
Calculating the mean volume of carbon dioxide collected in a set amount of time increases accuracy and reliability of the results, as the experiment is being replicated in the process. Nevertheless, one cannot ever place complete confidence in the accuracy of results achieved in this way as calculating the mean involves using even anomalous results. Therefore, the occurrence of errors remains possible. Error bars are used to show how much uncertainty or error the mean calculated presents. It is the mean value which is plotted on the graph, therefore, error bars were drawn on each point on the graphs for the actual experiment. An example of this can be seen below:
Table to show the amount of carbon dioxide produced in 20 minutes at 40ºC using the method for the actual experiment:
The mean at the two-minute interval= cm³ to 1 d.p.
The highest value of the three results at the two-minute interval=
The lowest value of the three results at the two-minute interval=
The degree of uncertainty can be worked out as shown below:
Highest value result – mean = + error
Lowest value result – mean = - error
The ‘true value’ for the amount of carbon dioxide collected lies somewhere between the error bars. The precision and reliability of the results is shown by length of the error bars. Long error bars indicate a great range within the results, which corresponds to a large uncertainty, or error within the results making them unreliable. Shorter error bars indicate a small range within the results, which corresponds to a small uncertainty, or error within the results making them more accurate and reliable.
The rates of reaction were calculated from each graph drawn using the results from the actual experiment. Drawing a tangent to the curve at any point where the curve was smoothly drawn did this. The gradient of this tangent was calculated using the following calculation: (change in y ÷ change in x)
Initial rates of reaction
The results that were recorded after 10 minutes at 60°C proved to be very significant. The rate of reaction being zero, illustrates that the reaction had stopped, which in turn demonstrates that the enzymes have been denatured.
Trends in the results
By looking at the graphs, it can be seen that in general, as the temperature increased, the volume of carbon dioxide gas produced by the yeast solution increased. Increasing the temperature speeds up the respiration, because the glucose particles have more energy, and they will collide with the active site of the yeast particles with greater frequency and force. Therefore, the chances of them forming enzyme-substrate complexes with the yeast cells increase (Biology Foundation 5.1.3 Enzymes). Enzymes involved in glycolysis act as biological catalysts, speeding up the rate of reaction from which glucose is converted to carbon dioxide. An increase in enzyme-substrate complexes formed with such enzymes, leads to an increase in the rate at which acetyl coenzyme A is fed into the Krebs’ cycle. Therefore, there is an increase in the rate of reaction. This is indicated by an increased volume of carbon dioxide gas produced and collected within the set amount of time. When anaerobic respiration is taking place, an increase in enzyme-substrate complexes formed with enzymes involved in the production of ethyl alcohol from glucose, leads to an increase in the rate at which pyruvic acid is converted first into acetaldehyde with the release of a molecule of carbon dioxide and then into an ethyl alcohol. Therefore, there is an increase in the rate of reaction. This is indicated by an increased volume of carbon dioxide gas produced and collected within the set amount of time. (Central Concepts 5.4.1 – Energy and Respiration).
Conclusions
Evaluation
Suitability
I feel that the procedure used was suitable enough for what I was trying to find out ,as it allowed a range of results to be found that could then be compared.. The simplicity of the experiment allowed procedures to be replicated, producing more reliable results. Results were numerical and so could be analysed and compared in several ways to solve the problem presented by this experiment, for example, a graph could be plotted.
Anomalous results and suggested reasons for anomalous results
When looking at the graph drawn, it is evident that these results do not follow in the same trend as the other results. The trend of the graph was that This illustrates that an error had been made, as if the results had followed in the trend of the graph, the percentage change in mass for 1.0M would have been less than that for 0.9M. This inaccuracy in results may have been due to inaccurate balance readings. It may have also been a result of inaccurate measurements in length when
Accuracy of measurements
My scientific knowledge shows that my results should primarily illustrate an increase in the mass of potato chips up to a certain point. At this point there should be no water going in or out of the potato cells, therefore no increase in the mass of the potato chips. Subsequent to this stage there should be a continuous decrease in the mass of the potato chips. My results almost strictly follow this pattern. This leads me to believe that the degree to which measurements and observations made approached a ‘true’ value was of a high extent. Therefore I can say my results were sufficiently accurate. Yet, there were still certain inaccuracies made. This was due to limitations in experimental procedures and equipment. One of these limitations was that measurements had to be made using 15cm rulers. This meant that they could only be used to measure to the nearest millimetre. Difficulties faced by struggling to read unclear markings on the ruler lead to inaccuracy in recorded measurements. Derived values such as rates of reaction were also inaccurate, as these invalid measurements were used in calculating them.
Suggested improvements for decreasing limitations
- Control could have been used to show that only yeast cells respire.
- In order to improve accuracy of measurements of potato chips, a clearly marked instrument that has a smaller scale could be used.
- A graph should have been drawn of the results found in the preliminary experiment. This would have allowed an earlier recognition of the potato cells having a water potential corresponding to that of a sucrose solution of molarity 0.3M. Then, in the actual experiment this molarity could have also been used to find a more accurate water potential of the potato cells.
- A higher volume of solution should be used in each boiling tube. This would guarantee that the potato cubes will be fully immersed in the solution. As all surfaces of the potato cubes will be immersed in the solution more of a fair test will be achieved.
Reliability of results
Overall, I believe that the results obtained were reliable. This was achieved by replicating the experiment three times for each temperature. Controlling variables to ensure a fair test, such as using equal volumes of yeast and glucose solution in the conical flask in each experiment, also increased reliability and precision of results. The reliability of a set of measurements depends on replicating the measurements and the experiment was replicated three times per solution.
Main sources of error
A factor that caused errors in results is evaporation of carbon dioxide gas through the connections between the apparatus. This would have resulted in a decrease in the volume of carbon dioxide gas recorded. The distortion of measurements in volumes of carbon dioxide gas produced caused by evaporation therefore means that the results are invalid. Any calculations made for analysis using these results would also be invalid and unreliable.
Effect of limitations and errors on validity of conclusions
Limitations in the experimental procedure used and errors lead to inaccurate results being measured. These inaccurate results were then used to derive values such as percentage change in mass, which were also concluded to be invalid as a result. Therefore, the conclusions are not very valid as a high measure of confidence could not be placed in the conclusion.
In general, the results were obtained were reliable as the individual experiments were replicated three times to produce three sets of results. This increased reliability and the fact that the results obtained from these sets were all very similar shows that the measurements made were accurate. For example, at 40°C at two minutes the three experiments had produced: 7cm³ of carbon dioxide, 8cm³ of carbon dioxide and 8cm³ of carbon dioxide. All the results obtained from the investigation were within 2cm³ or 3cm³ of each other. None of the results were wildly different from each other. However upon plotting graphs and analysing it can be seen that the first few results obtained from the experiments carried out at 30°C are anomalous as when plotted they do not produce a smooth curve.
E5.b
Main sources of error
The main sources of error were from temperature fluctuations during the experiment, the unsteady flow rate of carbon dioxide gas after it had been produced, from the conical flask to the gas syringe. This resulted in the build up of gas between the times measurements were made and recorded. This distorted the pattern of the results. Moreover the possible loss of gas through the connections between the apparatus is a main source of error. In addition for some of the experiments the reactions took some time to get going and therefore expected results were not obtained. This caused anomalous results.
E7.b
Validity of conclusions
Due to the temperature fluctuations and the unsteady flow rate of the carbon dioxide some of the results obtained were distorted and therefore invalid. Also, the possible loss of carbon dioxide gas made some of the results obtained inaccurate. These factors decreased the reliability of the results and consequently the reliability and accuracy of the deductions and calculations made during analysis. With this in mind, the graphs plotted and rates of reaction calculated were less reliable and accurate and so the conclusions drawn based on these values were in turn distorted, in other words they were less valid. However, the conclusions made as a whole were valid but there are still some uncertainties associated with some of the anomalous results.
· The number of times that I shake the test tube. (This has to be controlled because the shacking of the test tube activates the yeast. If I shake the yeast it becomes more active thus it will give off more hydrogen, which will affect the time taken for the colour of the TTC to change. I will control this by shacking the test tube before the experiment and every 60 seconds by placing the bung on the test tube and shake it 10 times by turning it upside down. I will then remove the rubber bung).
· The batch of yeast (This has to be controlled because different yeast batches have different activities. This will affect my experiment because if 1 batch of yeast was active then rate of reaction would be faster and if another batch was not very active then my rate of reaction will be slow giving me inaccurate results. I will control this by trying to complete my experiment in one lesson so I can use the same yeast each time).
We had to keep the beaker rapped in tin foil so that the temperature would stay as close as possible to the temperature that we were measuring. The tin foil acts as an insulator and keeps the heat inside the water.
the same thermometer during the whole experiment the results would be inaccurate, as I would have to wait for the thermometer to reach the certain temperatures again.
I made sure that I used the same amounts of yeast accurately, so that I could get the most accurate results. Upon looking back at the adjustments made, I believe that I am confident in saying that, I made sure that my experiment was as fair as I could possibly make it.