However, I will not be using the bubbles method in my main experiment as it is far too inaccurate and does not provide the right results. Instead I will be using a method of displacement. As in the preliminary work, there will be a second test tube of water, but instead of counting the bubbles in this water, I will be measuring the amount of water displaced by the gas (carbon dioxide) coming from the yeast.
I expect to see two types of respiration from the yeast; aerobic (with oxygen) and anaerobic (without oxygen). Here are the two equations representing this:
Aerobic:
Glucose + Oxygen Energy + Carbon Dioxide + Water
C6H12O6 + 6O2 Energy + 6O2 + H2O
Anaerobic:
Glucose A bit of energy + Ethanol + Carbon Dioxide
C6H12O6 A bit of energy + 2C2H5OH + 2CO2
I am going to use the method of displacement for my main experiment because it provides much more accurate results, and as shown in the equations, yeast produces the gas carbon dioxide when it respires. I expect to see both types of respiration because, the yeast at the top of the test tube will have access to oxygen, and can respire aerobically, while underneath, the yeast has no access to oxygen and will have to respire anaerobically. The carbon dioxide produced from the yeast’s respiration will displace some of the water in the test tube, and I will be able to measure the amount of water displaced to get the results for my experiment.
Prediction
From the results of my preliminary work, I predict that between 20o and 40oC, the yeast enzymes will be respiring fastest because enzymes work best at room temperature. I also predict that as the temperature goes up, the respiration will get slower and slower because the enzymes will start to denature (the yeast enzymes stop working) at higher temperatures. This is shown in the graph on the next page:
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40o is the optimum temperature because it is just the right temperature for the enzymes to be working at maximum rate. Yeast is an enzyme, which means that it is also a catalyst. Enzymes work using the ‘lock and key’ theory, where the ‘key’ fits into the active part of the enzyme (the ‘lock’) and the reaction takes place. The key then unlocks to form one or two more new substances and the enzyme is ready to bind with another of these substances. An enzyme can only bind with a substance that fits the shape of the active part of the enzyme, so, because the enzymes are sensitive to higher temperatures, the active site on the enzyme changes shape so much that binding can hardly take place. This is the denaturation. Up until a temperature of about 50o, as the temperature rises, the yeast works at a faster rate with the heat, because catalyst speed up reaction rate, but it is heat that speeds up the catalyst. This is shown in the diagram below:
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Apparatus
- A test tube
- Two beakers
- Water
- Yeast
- Stopwatch
- Delivery tube
- Thermometer
- Measuring cylinder
- 35ml syringe
Method
- Heat up the water in the water bath to the desired temperature, using a tap water to cool, and a thermometer to measure the temperature.
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Fill a beaker with water of the desired temperature (30, 40, 50, 60, 70oC) measuring the temperature using a thermometer.
- Fill the test tube with 35ml of yeast using the syringe and place into the beaker of water.
- Measure the equilibration time (4 minutes) using a stopwatch and check using a thermometer.
- During the 4 minutes, fill a measuring cylinder to the top mark.
- When the equilibration time is up, place the bung end of the delivery tube into the test tube of yeast, and the other end of the delivery tube into the measuring cylinder. Turn the measuring cylinder upside-down taking care not to lose water, and place into the other beaker, filled with cold water.
- Measure 2 minutes using the stopwatch, and take out the bung from the test tube of yeast.
- Measure the displacement of the water in the measuring cylinder.
- For each of the temperatures, repeat the experiment twice.
See experiment diagram next page.
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Preliminary Diagram
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Main Experiment Diagram
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Results
Equilibration time used= 4 minutes
See graph on next page
Conclusion
From the graph we can see that at 30oC, the displacement (respiration indicator) is 2.25ml, and this is quite a low level of respiration. The respiration reaches its optimum temperature at 40oC and the level of respiration falls as the temperature rises and the enzymes denature. At 70oC however, the highest temperature used, there is still some respiration going on, and this was not expected, as it was thought that the enzymes would have completely denatured (stopped working).
At temperatures around 40oC, the optimum temperature, there are more collisions between the enzymes and glucose molecules, and the molecules are slowly broken down and used for respiration. When the temperature is too high, such as 70o, the enzymes start to denature, which means that there are less and less collisions with the glucose molecules, thus less respiration.
Enzymes work using the ‘lock and key’ theory, where the ‘key’ fits into the active part of the substrate, and a reaction takes place. The key then unlocks to form one or two other substrates, and the enzyme is ready to bind again with one of these new substrates. An enzyme can only bind with a substrate that fits the shape of the active part of it, so, because the enzymes are sensitive to higher temperatures (in my graph any temperature below around 50oC) the active part of the enzyme changes shape so much that the binding can hardly take place. This is the denaturation, and it also means less respiration. As the temperature rises to 40oC, the yeast enzyme works at a faster and faster rate, because it is a catalyst and therefore speeds up reaction rate. This ‘lock and key’ theory is shown in the diagram below:
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Analysis
My conclusion does support my prediction to quite a large extent. The graphs both show that I predicted correctly the optimum temperature would be 40oC, and also that after this temperature, the enzymes would start to denature. At 40oC, the enzymes are working the fastest, colliding with the glucose molecules, and breaking them down to be used for respiration. The only difference between the prediction and conclusion is that I predicted that the enzymes would have completely denatured by 70oC, when in fact there was still some respiration. This shows that my conclusion supports my prediction to quite a large extent.
