1. BIOLOGY CAPT Embedded Task
Lab Investigation: Yeast Population Dynamics

2. 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.

3. Use of Energy, Growth and Reproduction in Yeast
The yeast cells undergo cellular respiration! As they grow, more cells make more CO2!
SUGAR
The process repeats producing more yeast cells and even more CO2 gas as more cells make more gas
Yeast Cell
Cell Division Occurs
ADD
SUGAR

4. 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.

5. 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

6. CO2 Collection Set-up Resulting Air Bubble
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.
Large Test Tube
Small Test Tube inverted and filled w/ yeast molasses solution

7. Collecting Data Students Collect CO2 Bubble Height Data for 5 Days
5mm
11mm
30mm
Students Collect CO2 Bubble Height Data for 5 Days
Day 1 Day 3 Day 5
Time in DAYS

8. 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

9. Sample Experiment: How does changing temperature
affect the yeast population size?

10. Sample Experiment: The Effect of Temperature on Yeast Population Size
Temp: 4°C (original bubble size: 5 mm)
Day 1 Day Day Day Day 5
Bubble Size mm mm mm 6mm mm
Temp: 24°C (original bubble size: 4 mm)
Day 1 Day Day Day Day 5
Bubble Size mm mm mm mm mm

11. Sample Experiment: The Effect of Temperature on Yeast Population Size
Temp: 30°C (original bubble size: 5 mm)
Day 1 Day Day Day Day 5
Bubble Size mm mm mm mm mm
Temp: 44°C (original bubble size: 4 mm)
Day 1 Day Day Day Day 5
Bubble Size mm mm mm mm mm

12. Raw Data: CO2 Bubble Height from Yeast Grown at Various Temperatures
Day 1
0.0 10 8 2
0.5 18 20 14 3 29 35 24 4 1 34 45 28 5 38 53 30
Bubble Height measured in mm.

13. Analysis Questions
Directions: On a separate sheet of paper, answer the following questions in complete sentences.
Identify the following for this experiment:
Problem statement
Independent variable
Dependent variable

14. 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?

15. 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.