Burping Yeast: An Investigation of Cellular Respiration
Student Materials
Introduction........................................................................................................... 2
Lab Protocol.......................................................................................................... 5
Data Collection Worksheet .................................................................................. 7
Pre-Lab Questions................................................................................................ 8
Post-Lab Questions and Analysis ....................................................................... 9
Students
You should read the Introduction and Lab Protocol and then answer the Pre-Lab Questions.
Also be sure to answer the questions that are embedded in the Introduction. Completion of
the questions will help you understand the concepts and procedures of the lab. Once you
have completed the lab, answer the Post-Lab Questions and Analysis.
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Burping Yeast: An Investigation of Cellular Respiration
Introduction
All living things have a number of characteristics in common. Whether a single-celled bacteria or a human, all
living things grow, develop, sense their environment, and work to maintain stable internal conditions. Each of
these activities require energy, which brings us to another shared characteristic of all life: all living things convert
the energy stored in food into cellular energy. Of course, what constitutes food for a plant or bacteria is a bit
different from what you consider food, but no matter what form the “food” takes, it almost always comes down to
the same basic building block—the simple sugar glucose (C6H12O6). The process of converting energy stored in
the chemical bonds of glucose into the chemical bonds of the molecule ATP is called cellular respiration.
During cellular respiration, glucose is broken down by a series of chemical reactions. As the chemical bonds of
glucose are broken, the energy and electrons released can be used to produce ATP, or adenosine triphosphate.
ATP, like glucose, is an energy storage molecule, but unlike glucose, cells can use ATP directly. ATP is used by
cells to do everything from replicating DNA to transporting some molecules across the cell’s membrane. In
humans, it is ATP that makes our hearts beat, our muscles contract, and our lungs fill with air—in short, ATP
provides the energy that makes cells “go”—no ATP, no life.
Most cellular respiration occurs in the presence of oxygen and is called aerobic (“with oxygen”) cellular
respiration. When oxygen is not available some cells (yeast and overworked muscles cells, for example), can
undergo an anaerobic (“without oxygen”) process called fermentation. Both pathways begin with a set of
reactions known as glycolysis, which transforms glucose to the 3-carbon molecule pyruvate. In the aerobic
pathway, the pyruvate is further broken down in the reactions of the Krebs cycle.
For most biochemical reactions (even spontaneous reactions) to occur in a time frame relevant in a cell, an
energy barrier called the energy of activation must be overcome. Enzymes are specialized proteins that speed
up chemical reactions by lowering the activation energy required. Enzymes are affected by the conditions of
their environment, including pH and temperature. Each enzyme is specific for a single reaction and is effective
within a narrow range of conditions. If conditions vary too much from this range, then the enzyme cannot work.
Where do the carbon atoms from pyruvate end up? You know the answer to this question! Take a
deep breath and hold it. Now blow out. What are you exhaling? _______________________
Electrons released from the breakdown of glucose (and pyruvate) following the Krebs cycle are carried to the
electron transport chain.
What molecule accepts the electrons from glucose? You know the answer to this question, too!
Take a deep breath. What did you just inhale? ____________________________________
When oxygen is absent, where do the electrons released from the breakdown of glucose and pyruvate go? They
are given back to pyruvate, generating ethanol and CO2 (in the case of yeast) or lactic acid (in the case of
muscle cells).
Why does respiration or fermentation by yeast cause bread dough to rise? ______________
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Both aerobic respiration and fermentation produce ATP. The aerobic pathway (via glycolysis, the Krebs cycle,
and electron transport chain) produces much more ATP than fermentation per molecule of glucose—more than
30 ATPs for the aerobic pathway and just 2 ATPs (from glycolysis) for fermentation. Figure 1 on the following
page shows the major sets of reactions involved in fermentation and aerobic respiration.
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Figure 1. Overview of the ATP production during cellular respiration. The aerobic cellular respiration
pathway (glycolysis + Krebs cycle + electron transport chain) generates 25-32 molecules of ATP per molecule of
glucose. By contrast, fermentation generates just 2 ATPs per molecule of glucose.
The overall equations for both aerobic cellular respiration and alcoholic fermentation (the type of fermentation
carried out by yeast) are below.
Aerobic Cellular Respiration:
C6H12O6 + 6O2  6CO2 + 6H2O + energy
Glucose + oxygen  carbon dioxide + water
Alcoholic Fermentation:
C6H12O6  2CO2 + 2C2H5OH + energy
Glucose  carbon dioxide + ethyl alcohol
In the lab today you will investigate the breakdown of glucose by baker’s yeast (Saccharomyces cerevisiae).
Like all eukaryotes, baker’s yeast cells contain a nucleus and other membrane-bound organelles including
mitochondria, the site of aerobic cellular respiration in eukaryotes. But baker’s yeast is special in that it is a
facultative anaerobe, which means that under some conditions, such as low oxygen concentration and/or high
glucose concentration, the cells can switch from aerobic respiration to fermentation. Notice from the equations
that as the cells carry out both forms of cellular respiration, they break down glucose and release carbon dioxide
gas. In this lab you will compare rates of under different conditions by counting the number of carbon dioxide
bubbles released by the yeast over 10 minutes.
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Burping Yeast: An Investigation of Cellular Respiration
Lab Protocol
Materials:
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3  600-mL beakers
3  test tubes
1  test tube rack
3  transfer pipettes
3  metal hex nuts
3  2.0-mL microcentrifuge tubes
1  p1000 micropipettor and pipette tips
1  thermometer
timer
calculator
permanent marker (such as Sharpie)
glucose solution
yeast solution
Procedure:
Prepare your test tubes:
1. Using the p1000 micropipettor, add 500 L of yeast solution and 500 L of glucose solution to each of three
microcentrifuge tubes. Caution! To avoid contamination, be sure to use a new tip for each solution. Cap the
tubes and gently mix the solution by tapping the bottom of the tube with your finger.
Set up your respirometers:
2. When preparing the respirometer, it is very important that you get all of the yeast-glucose solution into the
bulb of the transfer pipette. Follow the steps below to deposit the solution into the bulb of a transfer pipette.
Important! Each student in the group should set up a respirometer, so there will be three total per
group.
2.1. Obtain a clean transfer pipette.
2.2. Completely depress the bulb of the pipette.
2.3. Slowly draw up the entire 1.0 mL of yeast-glucose solution from one
tube into the pipette.
2.4. Flip the pipette bulb side down.
2.5. Release the bulb to draw the solution into the bulb.
2.6. Slide a metal hex nut over the shaft of the transfer pipette loaded
with the yeast-glucose as shown in Figure 2 and place the
respirometer (still inverted) into a test tube rack. Use an additional
hex nut if needed.
Figure 2. Respirometer. Carefully following steps
2.1–2.6 to ensures that all of your yeast-glucose
solution is loaded into the bulb of the transfer pipette.
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Set up the experimental conditions and control:
3. Obtain 3  600-mL beakers and label them each with the permanent marker A, B, and C. Fill each one up,
almost to the top, as outlined below:
 Beaker A: fill with water from source A (coolest temperature water)
 Beaker B: fill with water from source B (medium temperature water)
 Beaker C: fill with water from source C (warmest temperature water)
4. Fill a test tube with water from one of each of the three beakers (A, B, and C) and place it in the
corresponding beaker. Example: Fill a test tube with water from Beaker A and then place it in Beaker A. Repeat
for the remaining two beakers.
5. Slowly lower a respirometer (still inverted) into each of the test
tubes in the three beaker water baths, as shown in Figure 3.
6. Set your timer for four minutes. Do NOT record any data until
timer goes off. This time allows yeast-glucose solution in the
respirometer to reach the same temperature as the surrounding
water. While waiting for the four minutes to elapse, assign each
your group members to a beaker for data collection (Beaker A, B,
C) and place a thermometer into each beaker.
the
7. Once the timer goes off, record the starting water temperature
your beaker in the appropriate place in data collection table (next
page). Important! Be sure to record this and all data you collect
the column that corresponds to your assigned beaker.
in
of
or
in
Collect Data
8. Set the timer for one minute.
9. When all groups members are ready, start the timer and begin
counting the number of bubbles that rise from the tip of your
assigned respirometers (A, B or C). Important! Be sure to watch
carefully—it is easy to miss the small bubbles! When the timer
goes off, record your data in the data collection table. Once
everyone is ready, reset and restart the timer and start your
count again. Repeat for 8 more times for a total of 10 1-minute
intervals.
Figure 3: Complete set-up.
The respirometer has been placed into a test
tube containing water from its associated
beaker, and then the test tube has been
placed into the beaker.
Think about it: What is the purpose of the
metal nut on the respirometer?
10. At the end of the 10-minute period, calculate the total number of bubbles you observed and record the data
in the appropriate place in the data collection table.
11. Complete your data table by copying the data recorded by each of your other group members.
12. Record any other observations you make during the experiment.
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Burping Yeast: An Investigation of Cellular Respiration
Data Collection Worksheet
Name:
Other group members:
Class/period:
Date:
Assigned Beaker (circle one): A
Time
(After 4 minute
equilibration time)
B
C
Number of Bubbles/Minute
Beaker A
Beaker B
Beaker C
coolest-temperature
medium-temperature
warmest-temperature
Starting Temp: _____C
Starting Temp: _____C
Starting Temp: _____C
Minute 1
Minute 2
Minute 3
Minute 4
Minute 5
Minute 6
Minute 7
Minute 8
Minute 9
Minute 10
Total number
of bubbles
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Burping Yeast: An Investigation of Cellular Respiration
Pre-Lab Questions
Directions: After reading through the introduction and protocol for the Burping Yeast lab, answer the questions
below.
1. Understanding Why is cellular respiration important to all living things?
2. Remembering What is the difference between glucose and ATP in terms of the cell’s ability to use it?
3. Applying Like baker’s yeast, human muscle cells can perform fermentation. Explain why muscle cells are
more likely to ferment during strenuous exercise than when at rest.
4. Remembering What is the role of enzymes in cellular respiration?
5. Remembering How will you measure the rate of cellular respiration in this experiment?
6. Applying Propose a hypothesis about how temperature will affect the yeast’s rate of cellular respiration.
7. Applying What are the dependent and independent variables for this experiment?
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Burping Yeast: An Investigation of Cellular Respiration
Post-Lab Questions and Analysis
Directions: After completing the Burping Yeast lab, answer the questions below.
1. Applying Why was counting the number of bubbles an appropriate way to measure the rate of cellular
respiration occurring within the yeast?
2. Analyzing In Excel, or by hand, generate a bar graph to compare the total number of bubbles counted for
each of the three beakers. Remember to label all axes and include an appropriate title.
3. Analyzing Calculate the average number of bubbles per minute for each beaker and record the information
below.
Beaker A:
Beaker B:
Beaker C:
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4. Analyzing In Excel, or by hand, create a second bar graph (like that for #2 above) that includes both the total
number of bubbles/10 minutes AND the average number of bubbles/minute for each temperature. Remember to
label all axes, include an appropriate title, and label or include a key to indicate which bars represent the total
number of bubbles and which represent the average number of bubbles.
5. Analyzing What did the results indicate about the effect that temperature has on the rate of cellular
respiration in baker’s yeast?
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6. Applying Why did the temperature affect the rate of cellular respiration?
7. Creating Do you think the first organisms on Earth used an aerobic cellular respiration pathway or an
anaerobic cellular respiration pathway? Explain your answer. Hint: The first organisms on Earth were not
photosynthetic. Where does the oxygen in Earth’s atmosphere come from?
8. Applying Bread dough is usually made using flour, water, salt, and sugar. You have learned that it will rise to
a fluffy ball when baker’s yeast is added and uses the sugar for fermentation.
a. Do you think that bread dough made without sugar would rise? Explain your reasoning.
b. Bread dough made with wheat flour can double or even triple in volume, but bread dough made with
flour from other grains may not rise as much. Form a hypothesis that explains the difference.
9. Creating Under what pH and temperature conditions do you think a human muscle cell carries out cellular
respiration? What about a cell from the lining of the human stomach?
Important! Each group member should hand in a completed data collection worksheet and
set of answered questions.
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