Sarah Verdun
4/4/12
BSC 307
Alcoholic Fermentation in Yeast
Teacher Preparation Notes
Adapted from original teacher guide by Dr. Ingrid Waldron and Dr. Jennifer Doherty, Department of
Biology, University of Pennsylvania, 2011
Teaching Points
• If sugar is available but oxygen is not, yeast uses glycolysis to metabolize the sugar and produce
ATP, followed by alcoholic fermentation which results in the production of alcohol and carbon
dioxide.
• When more sugar is available, yeast has a faster rate of alcoholic fermentation.
• Experimental method (e.g. need for controls, replicates, change only one variable at a time)
Overview
Students will participate in a laboratory experiment that demonstrates alcoholic fermentation in yeast.
The students will mix yeast into a sucrose solution in an Erlenmeyer flask. The flask will be covered with
a balloon. The students will observe the CO2 produced by recording observations about the balloon and
about any bubbles produced immediately after the ingredients are combined, and again ten minutes
and twenty minutes after the experiment has begun. In this version of the experiment, different
concentrations of sucrose will be given to each student group. A comparison of class data will allow
students to determine how the concentration of sucrose affects the rate of fermentation.
Equipment and Supplies (For four groups, each with a different concentration of sucrose)
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Four 40ml Erlenmeyer flasks
A marker and tape to label the flasks
2 tsps Fast-rising highly active Baker’s yeast (1/2 tsp per pair of students)
Four 50ml graduated cylinders to measure sucrose solutions
Four ½ teaspoon measures for yeast
Four balloons that fit snugly over the flasks
4 rulers to measure in millimeters
Warm sucrose solutions: 1%, 5%, 10% (50 ml of each)
o 1%: add 1 g of sugar to every 99mL of water
o 5%: add 5g of sugar to every 95mL of water
o 10%: add 10g of sugar to every 90mL of water
Warm tap water (about 50mL) (This is the 0% sucrose solution)
Optional: tub of hot water to keep sucrose solutions and water warm.
Instructional Suggestions
The flow of this activity is as follows:
1. Students learn basic information about aerobic cellular respiration and alcoholic fermentation.
2. Students carry out an experiment to test whether sucrose concentration affects the rate of alcoholic
fermentation in yeast and analyze the results.
Review metabolism on page 1 of the Student Handout, as well as the introduction to the experimental
design on page 2 of the Student Handout. This can be done the day of the experiment or the day before
if needed.
Additional information on metabolism is included on the next page for the benefit of the instructor.
Warm the water and sucrose solutions before the lab period by placing them in a tub of hot water in
order to assure that the yeast metabolism will be rapid enough to produce good results in the 20 minute
observation period.
The rate of fermentation will be measured by assessing the amount of CO2 produced by two methods:
measuring the depth of the layer of bubbles trapped in the foam on top of the yeast solution and
observing balloons capping the flasks which catch the CO2 produced and expand.
For students to see the general trends and variability in the results in different replications, collect the
data on the depth of CO2 bubbles at 20 minutes from each student group. Display these data in a graph
with sucrose concentration (0%, 1%, 5% and 10%) on the X axis and depth of bubbles in millimeters on
the Y axis. In a class discussion of these data, ask students if they are aware of any differences in
methods that could have resulted in points that deviate from the general trend; this may help students
realize the importance of precision in experimental methods to achieve accurate results.
To encourage further discussion on the topic, ask students "What do yeast need ATP for?" Guide
students to understand that yeast are single cell fungi which absorb nutrients from their environment
(e.g. bread dough, grapes, tree bark). This question will give you the opportunity to review a variety of
cellular processes that use ATP including synthesis of macromolecules such as proteins and pumping
ions across the cell membrane.
Discussion of Metabolism – background information
The following coupled equations summarize the overall chemical changes that occur in aerobic cellular
respiration of glucose.
C6 H12O6 + 6 O2 ---->---->---->----> 6 CO2 +6 H2O
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energy
\/
n ADP + n Pi -- -- -- -- -- -- -- > n ATP (n <= 29)
In the first equation, the multiple arrows represent the multiple steps of glycolysis, the Krebs cycle and
the electron transport chain.
For aerobic cellular respiration, the number of ATP molecules generated for each glucose molecule is
variable and less than previously thought. A brief explanation is provided in "Cellular Respiration and
Photosynthesis -- Big Concepts, Misconceptions, and Classroom Activities" (available at
http://serendip.brynmawr.edu/exchange/bioactivities/cellrespiration).
These recent findings are interesting as an example of how science progresses by a series of successively
more accurate approximations to the truth.
The following coupled reactions summarize the chemical changes in anaerobic metabolism when
glycolysis is followed by alcoholic fermentation.
C6 H12O6 ---->---->----> 2 CO2 + 2 C2H5OH
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energy
\/
2 ADP + 2 Pi -------------- > 2 ATP
One possible question is why yeast can't use glycolysis without alcoholic fermentation when O2 is not
available. In the process of glycolysis, NAD+ is reduced to NADH. For glycolysis to continue, NADH must
be oxidized back to NAD+. When O2 is available as an electron acceptor,
NADH enters the electron transport chain and is oxidized to NAD+ in a process that contributes to the
production of ATP. When oxygen is not available, yeast use alcoholic fermentation in which the
pyruvate produced by glycolysis is converted to acetaldehyde (releasing CO2) and acetaldehyde reacts
with NADH to produce ethanol and NAD+.
The yeast which is used to make bread is Saccharomyces cerevisiae. This yeast is a facultative anaerobe,
which means that when oxygen levels are low or glucose levels are high, sugar is metabolized without
using oxygen, resulting in the production of a small amount of ATP, as well as carbon dioxide and
ethanol. As the bread bakes, the ethanol evaporates. Bubbles which contained carbon dioxide provide
the fluffy texture of bread. Saccharomyces cerevisiae and other members of the same genus are used in
making wine and beer, where, obviously, the production of alcohol is a major goal.
Student Sheet with added information to guide students
Experiment: Effects of Sucrose Concentration on the Rate of Alcoholic Fermentation in Yeast
1. Humans use yeast every day to make bread, wine and beer. What is yeast?
If you want to make your own bread, you can buy yeast in the grocery store. This yeast consists of little
brown grains. The little brown grains of yeast may not seem to be alive, but if you put them in water
with sugar, the yeast will take up the sugar and use the energy stored in the sugar molecules to make
ATP and carry out the processes of life. - Is yeast alive? -Is it a plant, animal, bacterium, fungi? -What
does it do?
2. What is sucrose? How does the yeast use this?
- Yeast can convert sucrose into glucose and use the glucose to provide the energy to make ATP.
3. In your experiment, you will grow yeast in a flask filled with water and sealed with a balloon. Are
these growth conditions aerobic or anaerobic?
- Initially it may be aerobic, but once the oxygen is used up the balloon will prevent more oxygen from
entering the system and the conditions will be anaerobic.
Under anaerobic conditions, yeast carries out glycolysis to produce ATP, followed by alcoholic
fermentation which produces ______CO2________ and ________water_________.
To measure the rate of alcoholic fermentation in yeast, we will measure the amount of bubbles in the
foam on top of the yeast solution and observe the balloons on top of the flask.
4. What are the bubbles in the foam? What is in the balloon? Where did it come from?
- CO2 from the fermentation carried out by the yeast
5. To test whether the concentration of sucrose affects the rate of alcoholic fermentation in yeast, our
class will measure the rate of CO2 production for 4 different concentrations of sucrose. Each group will
get one solution of sucrose. Complete the table to predict how much CO2 production you expect in
each case.
6. a. What is the independent variable in your experiment?
The amount of yeast; the time allowed for the experiment
b. What is the dependent variable in your experiment?
The concentration of sucrose
7. a. What is the control treatment in your experiment?
The control is the 0% sucrose solution.
b. What is the purpose of this control treatment?
This control creates a basis for comparison between the other concentrations of sucrose solutions.
8. While you're waiting for the 10 minute and 20 minute data collections, complete the following
questions.
- What is the main advantage of aerobic respiration?
- What is the main advantage of anaerobic fermentation?
9. What is fermentation?
10. Compare your results with your predictions in question 4. Did the amounts of CO2 produced at
different sucrose concentrations match your predictions? If not, how did the results differ from your
expectations?
11. Discuss your results with your group. What conclusions concerning the relationship between
sucrose concentration and the rate of alcoholic fermentation are supported by your results?
12. Compare your results with the class results or with the results of the group next to you. Are your
results generally similar? If there are any significant differences in results, what could be the reason for
these differences?
Use the following tables to compile class data.
Sucrose Concentration & Depth of CO2 bubbles
0 minutes
10 minutes
20 minutes
0% (plain water)
1% sucrose
5% sucrose
10% sucrose
Sucrose Concentration & Balloon Description
0 minutes
0% (plain water)
1% sucrose
5% sucrose
10% sucrose
10 minutes
20 minutes