Yeast Populations in a Closed Ecosystem

Yeast Populations in a Closed Ecosystem 9/15

Integrated Science 3

Redwood High SchoolName:Per:

Background

Yeast, Saccharomyces cerevisiae,are unicellular fungi. Because of their small size and rapid reproductive rates they are ideal organisms for studying the dynamics of populations. Humans also find them quite useful for a variety of purposes, including baking bread and brewing beer. In this lab, you will observe a flask population of yeast cells growing in a broth medium. This type of population is a closed population, whose changes are entirely dependent on a combination of birth rate and death rate. In contrast, open systems can also increase or decrease in size as organisms enter and leave. In nature, open populations are much more common. Conditions in a closed population are not as complex, and the study of population growth rates is easier than in open systems.

In this laboratory experiment, we are interested in comparisons of total yeast populations under different experimental conditions. As you will soon see, these actual populations will reach into the billions of individual organisms, far too many to be actually counted. As a result, population studies (as well as many other types of scientific work) rely on samples of the larger population to generate the experimental data. By making cell counts from small portions of the entire population, it is possible to accurately estimate the entire population. This method is called sampling. To assure the accuracy of these statistical estimates, it is necessary to closely follow standardized sampling procedures, including the averaging of results from a number of sample counts. The success or failure of this experiment and its data analysis will rest largely on your team's ability to follow the standard sampling protocols and perform the subsequent data calculations to arrive at a total population for your culture.

Experimental Design

Each team will receive a yeast culture and analyze yeast population growth over eight days. The culture consists of the following: 25 ml of sterile yeast medium (made from distilled water, dextrose sugar and peptone protein) and 0.1 ml of stock yeast culture. Dextrose is a nutrient source and peptone provides structural proteins for the yeast. This culture is grown in a sterile culture flask, plugged with cotton to permit the exchange of gas while preventing contamination by other organisms (primarily bacteria and fungus). The cultures will be grown under "room temperature conditions" in an incubator.
Each team will be assigned a culture with a different amount of dextrose sugar: 0%, 2%, 4%, 8% or 16%. The culture with 4% dextrose will serve as the control, as it contains the recommended amount.

Procedures

Complete an experimental organizer for this experiment.

Receive your yeast culture. Label the flask with team name, % dextrose and period. Record your team name (A or B) and your % dextrose here: ______
These cultures contain a fast growing population of living organisms, resulting in dramatic changes from one 24hr period to the next. Consequently, data must be collected almost every day for a total of eight days. Plan with the other members of your team for data collection on Wednesdays for periods 1-3, Thursday for periods 4-7, and on the weekend as instructed by your teacher. Only one member of each lab team needs to be present on the non-class data collection sessions.
Record your data in the provided data table for each day of data collection. Record class data on the provided data table for each day of data collection.

On a separate sheet of paper, create a Laboratory Notebook Entry for each day of data collection. Be sure to include drawings/diagrams and identify any procedural errors your team may have made. Attach your notebook entries to the back of this lab.

Sampling and Data Collection Protocol

*Stopping contamination by other microorganisms - organisms that will compromise your culture - is a major challenge when doing this type of work. Other microorganisms will also use the yeast medium as a source of nutrients; and in some cases devour the yeast. As a result, it is of utmost importance that you strictly follow all sterile protocols.

Go To The Designated Sterile Sampling Station:

a)Obtain a hemocytometer slide and cover slip. Place the cover slip on the hemocytometer slide.

b)Swirl your team’s flasks to mix the culture. This will evenly distribute the yeast.

c)Using a sterile dropping pipet, transfer 1 small drop from the culture flask under the coverslip of the slide. This drop will fill the space under the cover slip by capillary action. Return any culture medium left in the dropping pipet to the culture flask. Place the pipet in the used container. Quickly replace the cotton plug on your flask.

d)Find the yeast at 40X, then at 100X. Focus in on the hemocytometer slide at 100X. Adjust the stage so that the objective is oriented over the center of the slide as the circle is in the figure to the right.

e)Switch to 400X, and count the yeast at 400X using the procedure described in Sampling Protocol number 2.
Note: Yeast cells are difficult to see if the light is too bright (adjust the diaphragm to control the amount of light that reaches the slide).

Each group will count, and record, twelve grids from five specific boxes on the hemocytometer slide. Make certain you are observing yeast cells and not other material (like air bubbles). Count yeast in the inside and outside corners of the grids that correspond with 1a, 1e, 5a, and 5e on the figure to the right. Count the yeast in allfour corner grids of section 3c.

Average your team’s twelve counts. Record this value on your raw data table.
Average the Team A average with Team B average and record this value on your raw data table. In addition, report this value on the class data table.
Using class data average cell count values, calculate the following for all levels of the independent variable:

cells/ml:

grid volume = ______

population/culture:

Daily growth rate

Data and Results

Complete Data Table 1 with your lab group. Share your data with the other team in your I.V. leve (same dextrose percentage). Complete the lab calculations for your I.V. level for Data Table 2 and be sure to collect the rest of the class’ data each day. Construct a graph displaying the effects of time on yeast population growth. The graph will contain five curves, one for each level of the independent variable.

Experimental Organizer

Title (Describe the experiment by using the format, “The effect of ___ on ___”) :

Hypothesis (Make specific predictions about the experimental outcome using the format, “If…, then…”):

Independent Variable I.V. (What you will be testing.):

Category of I.V. (Identify if the I.V. is continuous or discontinuous.):

Levels (treatments) of I.V. (Identify the control and at least 2 additional I.V. levels.):

Number of Trials (Note the # of times each level of I.V. is tested.):

Dependent Variable (What you are measuring. Include all appropriate units.):

Category of Data (Identify if the data is qualitative and/or quantitative.):

Constants (List all conditions which are the same for each trial.):

Data Table 1: Raw Data of Yeast Population with ______% Dextrose

Yeast
Populations / Day 0: ______ / Day 1: ______ / Day 2: ______ / Day 3: ______
Team A / Team B / Avg. / Team A / Team B / Avg. / Team A / Team B / Avg. / Team A / Team B / Avg.
Count 1
Count 2
Count 3
Count 4
Count 5
Count 6
Count 7
Count 8
Count 9
Count 10
Count 11
Count 12
Average Counts
Yeast
Populations / Day 4: ______ / Day 5: ______ / Day 6: ______ / Day 7: ______
Team A / Team B / Avg. / Team A / Team B / Avg. / Team A / Team B / Avg. / Team A / Team B / Avg.
Count 1
Count 2
Count 3
Count 4
Count 5
Count 6
Count 7
Count 8
Count 9
Count 10
Count 11
Count 12
Average Counts

Data Table 2: Period _____’s Yeast Populations in a Closed Ecosystem

Yeast Populations / Day 0: ______ / Day 1: ______
0% / 2% / 4% / 8% / 16% / 0% / 2% / 4% / 8% / 16%
Average Count
Cells/ml
Population/
Culture
24-hour growth rate
Yeast Populations / Day 2: ______ / Day 3: ______
0% / 2% / 4% / 8% / 16% / 0% / 2% / 4% / 8% / 16%
Average Count
Cells/ml
Population/
Culture
24-hour growth rate

Data Table 2 (Continued): Period _____’s Yeast Populations in a Closed Ecosystem

Yeast Populations / Day 4: ______ / Day 5: ______
0% / 2% / 4% / 8% / 16% / 0% / 2% / 4% / 8% / 16%
Average Count
Cells/ml
Population/
Culture
24-hour growth rate
Yeast Populations / Day 4: ______ / Day 5: ______
0% / 2% / 4% / 8% / 16% / 0% / 2% / 4% / 8% / 16%
Average Count
Cells/ml
Population/
Culture
24-hour growth rate

Graph: ______

Discussion

Use the class data and your graph to evaluate the experimental hypothesis you generated at the start of this experiment. Answer the following prompts in complete sentences. Attach additional page if more space is needed.

Based on your data, describe what happened to the yeast population during the experiment. Be sure to refer to your graph and use experimental data in your answer.

______

Based on your data, explain why you got the results you did. Be sure to include a discussion of any experimental error. Do you believe your data was reliable? Why or why not?

______

Discuss the yeast experiment and explain how improvements could be made to the design and offer suggestions for further study.

______

How well did your team work together? Identify specifics of what went well, and places where your group could improve.

______