Instructor
Sugar Metabolism with Yeast
(Method 2–Ethanol Sensor)
OVERVIEW
Yeast can metabolize sugars in two ways, aerobically, with the aid of oxygen, or anaerobically,
without oxygen. In both cases, carbon dioxide, CO2, is produced. The rate that CO2 is produced
is referred to as the rate of respiration. If sugars are readily available, baker’s yeast
(Saccharomyces cerevisiae) prefers to metabolize glucose and other sugars anaerobically through
fermentation, producing ethanol in addition to CO2. This is commonly referred to as the Crabtree
effect.1
Figure 1
In the Preliminary Activity, your students will use an Ethanol Sensor to determine the rate of
fermentation of glucose by yeast. A student handout for the Open Inquiry version of the
Preliminary Activity can be found at the end of this experiment
During the subsequent Inquiry Process, your students will first find out more about sugars,
fermentation, aerobic and anaerobic respiration, and yeast using the course textbook, other
available books, and the Internet. They will then generate and investigate researchable questions
dealing with the fermentation of sugars by yeast.
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Fraenkel, D.G. Yeast Intermediate Metabolism, 1st ed. Cold Spring Harbor: Cold Spring Harbor Laboratory Press,
2011.
© Vernier Software & Technology
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LEARNING OUTCOMES
In this inquiry experiment, students will
Identify variables, design and perform the experiment, collect data, analyze data, draw a
conclusion, and formulate a knowledge claim based on evidence from the experiment.
• Obtain graphic representations of fermentation rate.
• Determine fermentation rate by yeast while using different sugars.
• Determine which sugars can be used as a food source by yeast.
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THE INQUIRY PROCESS
Suggested Time to Complete the Investigation
Inquiry Phase
Open Inquiry
Guided Inquiry
I
Preliminary Activity
35 minutes
35 minutes
II
Generating Researchable Questions
(Omitted in Guided Inquiry Approach)
15 minutes
0 minutes
III
Planning
10 minutes
10 minutes
IV
Carrying Out the Plan
50 minutes
50 minutes
V
Organizing the Data
10 minutes
10 minutes
VI
Communicating the Results
10 minutes
10 minutes
VII
Conclusion
5 minutes
5 minutes
MATERIALS
Make the following materials available for student use
Vernier data-collection interface
Vernier data-collection program
Vernier Ethanol Sensor
plumber’s tape
#6 split stopper
250 mL fermentation chamber
0.3 M glucose, fructose, and galactose
sugar solutions
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yeast suspension
magnetic stir bar
magnetic stir plate
Beral pipettes
four 15 mL conical tubes
0.3 M lactase enzyme solution
0.15 M glucose, sucrose, maltose, and
lactose sugar solutions
I. Preliminary Activity
This inquiry begins with an activity to reinforce prior knowledge of the use of Vernier
data-collection technology and to introduce a method for collecting fermentation rate data.
Sample Results
Figure 2 Fermentation of glucose by yeast
Answers to the Questions
1. Perform a linear fit on the graph. Record the slope of the line, m, as the fermentation rate (in
ppm/min).
Answers will vary. The fermentation rate determined in the Sample Results above is 135.7 ppm/min.
2. List three common sugars, other than glucose.
Answers will vary. Fructose (fruit sugar or levulose) and galactose are two common
monosaccharides. Sucrose (table sugar, contains a glucose and a fructose unit), maltose (malt sugar,
contains two glucose units), and lactose (milk sugar, contains a glucose and a galactose unit) are
three common disaccharides.
3. List three factors that could possibly affect fermentation rates of sugars by yeast.
Answers will vary. Some factors are pH, yeast strain, yeast concentration, sugar used, sugar
concentration, and number of sugar monomer units per molecule.
4. List at least one researchable question concerning the fermentation of sugars by yeast.
Answers will vary. See the Researchable Questions list below for some possible answers.
II Generating Researchable Questions
Note: Researchable questions are assigned by the instructor in the Guided Inquiry approach.
See page xiii in the Doing Inquiry Experiments section of Investigating Biology Through Inquiry
for a list of suggestions for generating researchable questions. Some possible researchable
questions for this experiment are listed below:
Recommended for Open Inquiry or Guided Inquiry (sample results provided)
How do the fermentation rates of various monosaccharides compare?
• How do the fermentation rates of various disaccharides compare?
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How do the fermentation rates of monosaccharides and disaccharides compare?
• Which sugars can be fermented by yeast?
• How does the fermentation rate of lactose solution containing lactase compare to the
fermentation rate of a lactose solution containing no lactase?
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Recommended for Open Inquiry or Guided Inquiry (no sample results provided)
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How does yeast concentration affect fermentation rate?
How does boiling the yeast affect fermentation rate?
How does salinity affect the fermentation rate of glucose by yeast?
How does the fermentation rate of 0.15 M sucrose (containing a glucose and a fructose unit)
compare to that of a solution 0.15 M in glucose and fructose?
How does the fermentation rate of 0.15 M lactose (containing a glucose and a galactose unit)
compare to that of a solution 0.15 M in glucose and galactose?
How does the fermentation rate of 0.15 M maltose (containing two glucose units per
molecule) compare to that of 0.30 M glucose?
Recommended for Advanced Students (no sample results provided)
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How does pH affect the rate of fermentation of glucose by yeast?
Can yeast metabolize artificial sweeteners?
How does sugar concentration affect fermentation rate?
How does the previous exposure of yeast to galactose affect galactose fermentation rate?
How does the previous exposure of yeast to sucrose affect sucrose fermentation rate?
Students should choose a researchable question that addresses the learning outcomes of your
specific standards. Be sure to emphasize experimental control and variables. (Instructors using
the Guided Inquiry approach select the researchable questions to be investigated by their
students. We encourage you to assign multiple researchable questions because this strategy
enhances student interaction and learning during phases IV–VII.)
III Planning
During this phase students should formulate a hypothesis, determine the experimental design and
setup, and write a method they will use to collect data. The plan should list laboratory safety
concerns and specify how they will be addressed during the investigation. Circulate among the
student groups asking questions and making helpful suggestions.
IV Carrying Out the Plan
During this phase, students use their plan to carry out the experiment and collect data. Circulate
among the student groups asking questions and making helpful suggestions.
V Organizing the Data
Student groups or individuals should organize their data using graphs, tables, and/or charts that
appropriately communicate the outcome of their experiment. They can prepare these using
Logger Pro, LabQuest, Graphical AnalysisTM for iPad®, PowerPoint, or other method.
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VI Communicating the Results
During this important and exciting phase of the inquiry process, student groups present their
research results using oral presentations, poster presentations, or journal club formats. Interaction
among groups researching different questions leads to a better understanding of the process of
fermentation.
VII Conclusion
Using your notes recorded during the Communicating the Results phase, summarize the group
results for the experiment and tell how they will fit into the upcoming instruction.
VIII Assessment
Scientific inquiry assessment may take on many forms.
SAMPLE RESULTS
Student results will vary depending on experimental design.
Comparing Sugar Fermentation Rates
Table 1: Monosaccharide Fermentation Rates
Sugar
Molecular formula
Respiration rate
(ppm/min)
fructose
C6H12O6
126.4
glucose
C6H12O6
137.1
galactose
C6H12O6
11.91
water (control)
H 2O
13.54
Figure 3 Fermentation rates of monosaccharides
This investigation addresses the question, “How do the fermentation rates of various
monosaccharides compare?” In this investigation, the fermentation rates of 0.30 M solutions of
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the monosaccharides listed were determined using the Preliminary Activity procedure. The yeast
suspension, made using Fleischmann’s Active Dry Yeast, was maintained at room temperature
after activation and prior to use.
Yeast cannot utilize all monosaccharides equally well. Fructose and glucose gave the largest
fermentation rates at 126.4 and137.1 ppm/min, respectively. Galactose, in contrast, is apparently
used to a minimal extent by yeast under the conditions of this investigation.
Table 2: Disaccharide Fermentation Rates
Sugar
Molecular formula
Fermentation rate
(ppm/min)
sucrose
C12H22O11
55.98
maltose
C12H22O11
43.00
lactose
C12H22O11
14.00
water (control)
H 2O
7.76
Figure 4 Fermentation rates of disaccharides
This investigation addresses the question, “How do the fermentation rates of various
disaccharides compare?” In this investigation, the fermentation rates of 0.15 M solutions of the
disaccharides listed were determined using the Preliminary Activity procedure. The yeast
suspension, made using Fleischmann’s Active Dry Yeast, was maintained at room temperature
after activation and prior to use.
Yeast apparently cannot utilize all disaccharides equally well. Sucrose and maltose gave the
largest fermentation rates at 55.98 and 43.00 ppm/min, respectively. Lactose, in contrast, is
apparently used to a negligible extent by yeast under the conditions of this investigation.
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Table 3: Fermentation Rates of Lactose With and Without Lactase
Sugar
Fermentation rate
(ppm/min)
lactose
14.00
lactose with lactase enzyme
57.99
water (control)
7.76
Figure 5 Fermentation rates of lactose with and without lactase
This investigation addresses the question, “How does the fermentation rate of lactose solution
containing lactase compare to the fermentation rate of a lactose solution containing no lactase?”
In this investigation, the fermentation rates of 0.15 M solutions of lactose and lactose in the
presence of lactase enzymes were determined using the Preliminary Activity procedure. The
yeast suspension, made using Fleischmann’s Active Dry Yeast, was maintained at room
temperature after activation and prior to use.
Yeast apparently cannot utilize lactose without first breaking it down with the lactase enzyme.
Lactose on its own had a fermentation rate of 14.00 ppm/min, while lactose in the presence of
lactase had a fermentation rate of 57.99 ppm/min.
TIPS
1. To prepare the yeast suspension, dissolve 7 g (1 package) of dried yeast for every 120 mL of
distilled water. Incubate the suspension in 37–40°C water for at least 10 minutes. After
activation, the yeast suspension can be maintained at room temperature. For best results,
prepare the yeast suspension at least one hour before use. The yeast suspension can also be
prepared 24 hrs before use. Prepare the suspension as directed above and then add 5 g of
glucose to the solution. Place the yeast suspension in a beaker and place on a magnetic
stirrer. Maintain the suspension at a constant stirring speed overnight.
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2. To ensure consistent yeast suspension concentration, make the yeast suspension available in
a beaker situated on a magnetic stirrer. Maintain a constant stirring speed as students remove
portions. Important: The yeast portions must be removed from the middle of the stirred
yeast source.
3. The normal operating temperature range of the Ethanol Sensor is 20–30°C. All of the Sample
Results were determined at room temperature. The Ethanol Sensor does not have temperature
compensation built in so all experiments should be done at the same temperature.
4. If suitable pipets and pipet bulbs are not available, graduated Beral pipets can be substituted.
5. The stopper included with the Ethanol Sensor is slit to allow easy application and removal
from the probe. Remove the stopper from the sensor sideways by pulling it off through the
slit, not by sliding the stopper off the bottom. Doing the latter will result in the tip coming off
and potentially being lost. When students are placing the probe in the fermentation chamber,
they should gently twist the stopper into the chamber opening.
6. The Ethanol Sensor relies on the diffusion of gases into the probe shaft. Students should
allow a couple of minutes between trials so that gases from the previous trial will have exited
the probe shaft.
7. Water can be removed from and added to a water bath with a 20 mL syringe or a small
dipper.
8. Preparation of solutions (prepare all solutions in distilled water):
0.30 M glucose requires 5.40 g of anhydrous glucose, C6H12O6, per 100 mL of solution.
Alternatively, 5.95 g of glucose monohydrate, C6H12O6•H2O, per 100 mL of solution.
0.30 M fructose requires 5.40 g of fructose, C6H12O6, per 100 mL of solution.
0.30 M galactose requirees 5.40 g of galactose, C6H12O6, per 100 mL of solution.
0.15 M glucose requires 2.70 g of anhydrous glucose, C6H12O6, per 100 mL of solution.
Alternatively, 2.97 g of glucose monohydrate, C6H12O6•H2O, per 100 mL of solution.
0.15 M lactose requires 5.40 g of lactose monohydrate, C12H22O11•H2O, per 100 mL of
solution.
0.15 M maltose requires 5.40 g of maltose monohydrate, C12H22O11•H2O, per 100 mL of
solution.
0.15 M sucrose requires 5.13 g of sucrose, C12H22O11, per 100 mL of solution.
9. More information about the sensor used in this Investigation, as well as tips for optimal
performance, can be found in the sensor’s user manuals available for download from the
Vernier web site, www.vernier.com/sensors.
10. The plans that your students submit for approval should list laboratory safety concerns,
including chemical safety concerns, and specify how they will address these safety concerns
during their investigations.
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