GHS Science
CAPT Embedded Task Review
Lab Investigation: Yeast Population
Dynamics
1
Introduction
Why Study Yeast Populations?
• Yeast is a good model organism because it is simple, but has many of the
same processes as more complicated cells.
• Yeast use simple sugars to perform cellular respiration and make ATP.
During cellular respiration, they release CO2 as a waste product, just like
animals and plants.
[sugar + O2  CO2 + H2O + ATP (energy)]
• Since more yeast cells will release more CO2, we can use the amount (or
volume) of CO2 volume as a measurement of the amount of yeast cells.
The amount of yeast cells will tell us the size of the population.
[Amount of CO2 ~ Population Size]
DID YOU KNOW? Yeast population variations produce flavor characteristics in
a wide variety of food products such wine, beer, and bread.
Use of Energy, Growth and Reproduction in Yeast
The yeast cells
undergo cellular
respiration! As they
grow, more cells make
more CO2!
Yeast Cell
A
D
D
S
U
G
A
R
Cell Division
Occurs
S
U
G
A
R
The process repeats
producing more
yeast cells and even
more CO2 gas as
more cells make
more gas
Your Task
• Determine how environmental conditions
such as pH, temperature, light, etc. can effect
the growth of a yeast population
• REMEMBER: we can use CO2 (carbon dioxide)
volume as a measurement of the size of the
yeast population in the test tube.
Measuring Yeast Population Growth by
CO2 Production
Next, add 1ml of yeast
mixed in water
Fill a small test tube with
35 ml of a 25% Molasses
solution.
Mix the yeast in the
solution by putting the
clean palm of your hand
over the end and
inverting it 5 times
CO2 Collection Set-up
Resulting Air
Bubble
Large
Test
Tube
Small Test
Tube
inverted
and filled
w/ yeast
molasses
solution
This is the double test tube set up
for collection of CO2. To obtain this
the lager test tube is placed over
top of the smaller filled test tube. It
is then quickly inverted so that the
larger test tube is open and the
smaller one is upside down. An air
bubble will be left at the top of the
tube.
Collecting Data
5mm
11mm
30mm
Day 1
Day 3
Time in DAYS
Day 5
Students Collect
CO2 Bubble
Height Data for 5
Days
Experimental Design
• You need to choose an environmental variable to
change during the experiment.
• Environmental variables that you can change: pH,
temperature, light, or concentration of molasses
(sugar)
• The initial bubble size is subtracted from the bubble
size each day to calculate the total volume of CO2
collected
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Sample Experiment:
The Effect of Temperature on Yeast Population Size
Temp: 4°C
Bubble Size
(original bubble size: 5 mm)
Day 1
Day 2
5mm
5.5mm
Temp: 24°C
5.5mm
Day 4
6mm
Day 5
6mm
(original bubble size: 4 mm)
Day 1
Bubble Size
Day 3
14mm
Day 2
22 mm
Day 3
33 mm
Day 4
38 mm
Day 5
42mm
Sample Experiment:
The Effect of Temperature on Yeast Population Size
Temp: 30°C
(original bubble size: 5 mm)
Day 1
Bubble Size
15mm
Day 2
25mm
Day 3
40mm
Temp: 44°C
50mm
Day 5
58mm
(original bubble size: 4 mm)
Day 1
Bubble Size
Day 4
12mm
Day 2
18mm
Day 3
28mm
Day 4
32mm
Day 5
34mm
Raw Data: CO2 Bubble Height from Yeast Grown at
Various Temperatures
Temperature
4° C
24° C
30° C
44° C
Day 1
0.0
10
10
8
2
0.5
18
20
14
3
0.5
29
35
24
4
1
34
45
28
5
1
38
53
30
Bubble Height measured in mm.
Analysis Questions
Directions: On a separate sheet of paper, answer the
following questions in complete sentences.
1. Identify the following for this experiment:
a. Problem statement
b. Independent variable
c. Dependent variable
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Analysis Questions
2. What would you consider the control in this experiment? Why?
3a. Identify two variables that were held constant in this experiment.
3b. Explain why it is important to hold these variables constant.
4. Why do you think the size of the bubble doesn’t continue to
increase at the same rate during day 4 and day 5?
5. Based on the data, what would your conclusion be for this
experiment?
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Definitions
Problem Statement: The problem that the scientist is trying to solve.
Hypothesis: An educated guess that answers the problem statement. Often, an “If…
Then…” statement is used. So, If I change the independent variable, Then the
dependent variable will change in this way.
Independent Variable: The variable in an experiment that the scientist purposefully
changes. This change should affect the dependent variable.
Dependent Variable: The variable in an experiment that changes and that the scientist
measures. This variable depends on the independent variable.
Control: The “baseline” that you can compare the other groups exposed to the
independent variable against. Used test how valid the independent variable is.
Controlled Experiment: An experiment in which all factors are kept constant except for
the independent variable.
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