Idea Transcript
Alcoholic Fermentation in Yeast – A Bioengineering Design Challenge1 I. Introduction 1. The ingredients for bread include flour, yeast, sugar and water. What makes the dough rise, so the bread will be fluffy instead of flat?
2. Examine the little dry grains of yeast that are used to make bread dough. Do you think that these grains of yeast are alive? Explain why or why not.
One way to test whether these little dry grains of yeast are alive is to test whether they can carry out alcoholic fermentation. Alcoholic fermentation is the main process that produces ATP in yeast cells. This figure shows that during alcoholic fermentation: The sugar glucose is broken down to the alcohol ethanol plus carbon dioxide. ATP is synthesized from ADP and P. Energy released by the first reaction provides the energy needed for the second reaction.
The pair of curved arrows represents coupled chemical reactions; the top reaction provides energy for the bottom reaction.
3a. Why is this type of fermentation called alcoholic fermentation?
3b. Why do cells need ATP?
3c. How can alcoholic fermentation result in the production of gas bubbles?
Alcoholic fermentation is a complex process that includes twelve different chemical reactions. Each of the twelve chemical reactions in alcoholic fermentation of glucose requires an enzyme. 4. What are enzymes? What does it mean to say that "a chemical reaction requires an enzyme"?
1
By Drs. Ingrid Waldron and Jennifer Doherty, University of Pennsylvania, Biology Department, © 2018. Teachers are encouraged to copy this Student Handout for classroom use. A Word file with the Student Handout and Teacher Preparation Notes with instructional suggestions and background information are available at http://serendipstudio.org/sci_edu/waldron/#fermentation .
II. Do the dry grains of yeast contain living cells that can carry out alcoholic fermentation? To test for alcoholic fermentation, you will put the little dry grains of yeast in sugar water and observe whether or not bubbles are produced. You will use controls to check whether any bubbles produced by yeast in sugar water are the result of alcoholic fermentation. These controls will test whether grains of yeast can produce bubbles by a simple physical process or a simple chemical reaction like the reaction between vinegar and baking soda. For one control, you will use definitely dead yeast cells that have been boiled so enzymes in the grains of yeast have been denatured and are inactive. You will compare bubble production by these definitely dead yeast cells vs. bubble production by the possibly living yeast cells in grains of yeast that have not been boiled. 5a. Can the definitely dead yeast cells carry out alcoholic fermentation of sugar to produce CO2? yes___ no ___ 5b. Explain why or why not; include enzymes in your explanation.
5c. You will put possibly living yeast cells in sugar water in one cup and definitely dead yeast cells in sugar water in another cup. How would you interpret each of these possible combinations of results? Possibly Living Yeast Cells
Definitely Dead Yeast Cells
Produce bubbles of gas
No bubbles
Produce bubbles of gas
Produce bubbles of gas
Interpretation
6a. As another control, you will compare bubble production by the possibly living yeast cells in sugar water vs. plain water without sugar. Can living yeast cells in plain water carry out alcoholic fermentation? yes ___ no ___ 6b. Explain why or why not.
6c. What combination of results would provide evidence that the little dry grains of yeast contain living yeast cells that can carry out alcoholic fermentation?
The sugar you will use is sucrose, the common sugar that people put in their coffee and use for baking. Sucrose is a disaccharide that yeast cells convert to glucose for alcoholic fermentation. When the yeast cells produce CO2, this gas is trapped in bubbles in a layer of foam. Your teacher will pass around a cup with a foam layer so you will know what it looks like. 2
In your experiment to test for alcoholic fermentation in yeast cells: Experimenter 1 will test for CO2 production by possibly living yeast cells in plain water. Experimenter 2 will test for CO2 production by possibly living yeast cells in sugar water. Experimenter 3 will test for CO2 production by definitely dead yeast cells in sugar water. Experimenter 4 will be in charge of timing and measuring temperature. Use the following procedure for your experiment: Experimenter 1
Experimenter 2
- Label your cups. - Add 80 mL of 35°C water.
Experimenter 3 - Label your cup. - Add 80 mL of 35°C dead yeast suspension.
- Tear a piece of scrap paper into quarters; each experimenter should take a quarter-piece of paper for the next step. Experimenter 1: - Weigh 4 g of yeast and put this on your piece of paper.
Experimenter 2: - Weigh 4 g of yeast and 0.5 g of sucrose and put these on your piece of paper.
Experimenter 4
Experimenter 3: - Weigh 0.5 g of sucrose and put this on your piece of paper.
- Bend your paper and pour the yeast and/or sucrose into your cup. - Stir vigorously with a plastic spoon for 1 minute. Smash any clumps of yeast and if necessary use your second spoon to scrape off any yeast that is stuck to the first spoon. - Put your cups in the warm water bath.
- Add 35°C water to the container for the warm water bath, just to the level of the liquid in the cups. Do not add more water or the cups will tip over! - Put the thermometer in the water in the bath. - Time 1 minute of stirring for Experimenters 1-3. - Measure the temperature of the bath. Record this starting temperature in question 7 on the next page.
- Time 10 minutes. - Make observations and record what you observe in the second and - At 8 minutes, measure and third columns of the table in question 7 on the next page. Do not bump record the ending the cups! temperature of the warm water bath. - At 9 minutes give a - At the end of 10 minutes, measure the depth of the foam layer at the warning. edge of the cup and record your results in the last column in the table - At 10 minutes announce in question 7. If the foam layer is not even, measure the depth at the that it is time to measure thinnest and thickest points and record both measurements and the the depth of the foam layer average. in the cups.
- Empty and clean the cups and bath. Clean up your workspace.
- Report the depth of the foam layer in each cup and the starting and ending temperatures to your teacher.
3
7. Record your observations in this table. Starting temperature _______° C Any bubbles right at the beginning?
Ending temperature _______° C Observations 1-9 min.
Depth of Foam Layer (mm) at 10 min.
Possibly living yeast in plain water (1) Possibly living yeast in sugar water (2) Definitely dead yeast in sugar water (3) 8. Compare your results in question 7 with your answers to questions 5c and 6c. Do your results support the conclusion that the “possibly living” yeast contained living yeast cells that could carry out alcoholic fermentation? yes ___ no ___ Explain your reasoning.
9a. Your teacher will provide the results for all the student groups in your class. Do the class results support your conclusions in question 8? If not, briefly summarize your conclusions based on the class results.
9b. Were the results similar for all the cups with “possibly living” yeast in sugar water? If not, what could be the reasons for any differences in results?
10. If you were going to repeat your experiment, how could you improve your methods to get more reliable and valid results?
4
III. Bioengineering Design Challenge When bakers make bread, they include flour with the yeast, sugar and water. The gluten protein in the flour gives elasticity to the dough and traps the CO2 bubbles produced by the yeast so the bread dough rises and the bread becomes fluffy. The fluffiness of the bread can be influenced by the relative amounts of yeast, sugar, water and flour, as well as other ingredients in the dough. The fluffiness of the bread can also be influenced by the temperature of the dough as it rises and how long the dough rises. Design Challenge. Jim Baker wants to make his bread as fluffy as possible without spending too much time waiting for the dough to rise. He has asked your class to find the amount of sucrose and temperature that produces the most CO2 in 10 minutes. He does not want his bread to be too sweet, so he doesn't want to use any more sucrose than needed for maximum CO2 production. To maintain good flavor and texture of the bread, he wants to keep the amount of yeast the same as in your experiment in Part II. Scientific Background. CO2 is produced by alcoholic fermentation, which includes multiple chemical reactions, each catalyzed by a different enzyme. Fortunately, you do not need to think about all of these chemical reactions and enzymes to predict the effects of changes in the amount of sucrose or temperature on the rate of CO2 production; you can get sufficiently accurate predictions by thinking about the expected effect for a single reaction catalyzed by a single enzyme. 11a. What happens to the rate of a chemical reaction catalyzed by an enzyme when the concentration of substrate increases? Give molecular explanations for the relationships between substrate concentration and rate of reaction. In the graph, draw a curve to show the expected change in the rate of CO2 production as the amount of sucrose increases.
11b. What happens to the rate of a chemical reaction catalyzed by an enzyme when temperature increases? Give molecular explanations for the relationships between temperature and rate of reaction. In the graph, draw a curve to show the expected change in the rate of CO2 production as the temperature increases.
Proposing and Testing Your Design Solution 12. Reread the Design Challenge above and list here the criteria for a good solution for this Design Challenge.
5
13a. Think about the best Design Solution for this Design Challenge. Based on your answers to questions 11a and 12 and the experiment in Part II, how much sucrose do you think will give the best Design Solution? 13b. What range of grams of sucrose in 80 mL H2O do you think should be tested to identify the best Design Solution? Explain your reasoning.
14a. Based on your answers to questions 11b and 12 and the experiment in part II, what temperature do you think will give the best Design Solution? 14b. What range of temperatures do you think should be tested to identify the best Design Solution? Explain your reasoning.
15. Your teacher will lead a discussion to develop a Class Investigation Plan. Suppose that one student suggests that each student group should decide which combinations of sucrose levels and temperature they will test. A second student argues that the different student groups should agree on the most important sucrose levels and temperatures to test and then cooperate to test each of these sucrose levels at each temperature. Which proposal will provide the most useful information for figuring out the best Design Solution? Explain your reasoning.
Record your group’s assigned temperature and assigned amounts of sucrose in question 16. Use the following procedure for your experiment. Experimenters 1-3 - Label your cups. - Add 80 mL of water at your assigned temperature. - Weigh 4 g of yeast and your assigned amount of sucrose onto a quarter-sheet of scrap paper.
Experimenter 4 - Add water at the assigned temperature to the container for the bath, up to the level of the liquid in the cups. - Put in the thermometer.
- Bend your paper and pour the yeast and sucrose into your cup. - Stir vigorously with a plastic spoon for 1 minute. Smash any clumps of yeast. - Put your cups in the bath.
- Time 1 minute of stirring for Experimenters 1-3. - Measure the starting temperature of the bath and record it in question 15.
- Do not bump the cups! At the end of 10 minutes, measure the depth of the foam layer and record your results in question 15. Measure the depth of the foam layer at the edge of the cup. If the foam layer is not even, measure the depth at the thinnest and thickest points and calculate the average. - Clean up.
- Time 10 minutes. At 8 minutes, measure and record the ending temperature of the warm water bath. At 9 minutes give a warning, and then at 10 minutes announce that it is time to measure the depth of the foam layer in the cups. - Report your results to your teacher.
6
16. Assigned Temperature ____° C Starting Temperature _____° C Ending Temperature _____° C Cup 1 Cup 2 Cup 3 Amount of Sucrose (grams in 80 mL of water) Depth of Foam Layer (mm) at 10 minutes 17. Your teacher will provide the information to fill out as many of the columns as possible in this table. (Each column gives the results from one student group.) Depth of Foam Layer (mm) Amount of Lowest Assigned Middle Assigned Highest Assigned Sucrose Temperature (______°C) Temperature (______°C) Temperature (______°C) (grams in 80 Test Test Test Test Test Test Test Test Test Average Average Average mL of water) 1 2 3 1 2 3 1 2 3 Lowest amount (_______ g) Medium amount (_______ g) Highest amount (_______ g) Starting temperature Ending temperature 18. Graph the results for the depth of the foam layer vs. the amount of sucrose at each temperature. Label the axes and provide a key that distinguishes between the results for tests 1, 2 and 3 in each graph.
Depth of Foam Layer (mm)
Amount of Sucrose
Highest Assigned Temperature
Depth of Foam Layer (mm)
Middle Assigned Temperature
Depth of Foam Layer (mm)
Lowest Assigned Temperature
Amount of Sucrose
Amount of Sucrose
7
19a. Interpret the results shown in the graphs in question 18. What effects did the amount of sucrose have on the amount of CO2 produced?
19b. Compare the effects of the amount of sucrose for tests 1-3 at each assigned temperature. Were the effects of the amount of sucrose consistent for these replicate tests? If not, what might account for any observed differences?
19c. Were the effects of the amount of sucrose consistent at different temperatures?
19d. Are the observed relationships between amount of sucrose and amount of CO2 produced compatible with your predictions in question 11a? If not, what scientific explanation could account for the observed results?
20. Graph the average depth of the foam layer vs. starting temperature for each amount of sucrose. Label the axes.
Starting Temperature
Highest Amount of Sucrose
Depth of Foam Layer (mm)
Middle Amount of Sucrose
Depth of Foam Layer (mm)
Depth of Foam Layer (mm)
Lowest Amount of Sucrose
Starting Temperature
Starting Temperature
21a. What effects did temperature have on the amount of CO2 produced?
8
21b. Were the effects of temperature consistent for different amounts of sucrose? If no, what could account for the variation?
21c. Are the observed relationships between temperature and amount of CO2 produced compatible with your predictions in question 11b? If not, what scientific explanation could account for the observed results?
22a. Summarize your conclusions concerning the optimum Design Solution. What advice would you give Jim Baker concerning the optimum amount of sucrose and temperature to maximize CO2 production in 10 minutes, but with no more sucrose than needed?
22b. Explain how your recommended Design Solution is supported by the experimental results.
22c. Describe any uncertainty you have about your recommendations.
23a. Describe any additional testing you would recommend to improve your design solution and/or increase your confidence in your proposed design solution.
23b. Would you advise continued testing with the same procedures or would you advise Jim Baker to begin testing with bread dough? What are the relative advantages and disadvantages of these two ways of testing different possible Design Solutions?
9