Pre-lab questions: Please define the following terms in your lab notebook. You should use these terms in the hypothesis, the analysis and the conclusion as you see fit.
Limiting factors:
Carrying capacity:
Exponential growth:
Logistical growth:
Density dependent factors:
Purpose: To study a population of yeast in a closed system over an extended period of time.
Question: What is the effect of time (5 days) on how the population of yeast grows if given only one dose of food?
Hypothesis:
Variables:
Groups:
Materials:
Sugar solution (30g/500ml) rehydrated yeast (3.5g/50ml)
2 small (10ml) test tubes
10ml pipette
1-50ml graduated cylinder
1 Test tube holder
1 eye dropper
Read the following procedure carefully. Highlight the validity measures within the procedure.
Procedure I: Culture preparation
1.
Label 2 small test tubes with your group name, period and letters A & B.
2.
Using the graduated cylinder, add 5 mL of water to test tube B and add 5 mL of sugar solution to test tube A. Do it in this order so that there is no sugar residue when measuring plain water.
3.
Add 1 drop of yeast solution to each test tube. Use your disposable pipette to mix well by slowly squirting solution in and out of disposable pipette.
4.
Cover test tubes with screw top and place test tubes in test tube rack at front of classroom.
The instructor will put the test tube rack in a cabinet overnight.
Procedure II: Sampling
1.
Mix the contents of the test tube evenly by slowly squirting solution in and out of a clean, disposable pipette. Do this to distribute the yeast throughout the liquid evenly before sampling.
2.
Using an eyedropper, place one drop of yeast solution from test tube A onto a clean microscope and use clean cover slip. Be sure to put the remaining yeast solution back into the test tube
and not down the drain!
***Caution: Iodine is dangerous – wear eye protection!!
Also, be careful not to mix up the eye droppers, putting the iodine dropper in your yeast culture will kill it!!!
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3.
Add a drop of Iodine to the yeast drop on the slide and cover it with a clean cover slip. Remove excess water from around cover-slip edge with a piece of paper towel if needed.
4.
On medium power, bring your slide into focus.
5.
Move to high power. Count the number of yeast cells that are in “contact” with the outside of your field of view only.
6.
Rinse your slide and cover slip and repeat steps 1 thru 5 for 2 more trials making sure you rinse
the eye dropper before each trial to avoid contamination.
7.
Repeat steps 2 thru 5 for test tube B except use a different dropper than you used for test tube A.
8.
Return your test tube to the storage rack and keep them in the dark! (Yeast can grow in the dark but many other organisms that might infect your sample cannot)
9.
Repeat the count at the same time each day for 5 days. Record all your data in your data table.
In journal, sketch a labeled drawing of the set up.
Data Collection:
 Create a data table that will accommodate your group’s data.
 Create a second data table to record the class data (done in class)
Each pair of people conducting this lab will report their average daily yeast cell counts and an entire class average should be included in this data table. It should look similar to your personal table but with a different number of rows and columns.
Graph and Data Analysis:
After 5 days, you will create a line graph to display both sets of data (personal and class). You should make one graph with 4 lines; two for your own average yeast population over 5 days (experimental and control) and 2 lines for the class average yeast population over the same period of time (experimental and control). Include a key (legend). Once you’ve complete the graph, answer the following questions:
1.
Describe the shape of the graphs. What do your graphs reveal about yeast population growth
(Is yeast population growth logistic or exponential? How do you know?)
2.
Explain the population growth between days 1 and 5. Why does the yeast population change in such a manner?
3.
What would you expect to happen to the yeast population in the future?
4.
Why is the class data more accurate?
5.
Why might our method of data collection be inaccurate? Hint: The dead yeast cultures look like just like the living ones.
6.
What would you expect to happen to the number of yeast as they run out of resources (food)?
Did you see this in your data? Why or why not.
7.
Why is it important that the yeast cultures were kept in the dark?
Conclusion:
Follow your general lab rubric and notes on the scientific process to complete this section. Make sure you keep question #6 in mind while writing whether your data supports or doesn’t support your hypothesis.
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Teacher notes:
In notebook:
Page 1 (flip page instruction sheet over)
Definitions:
Limiting factors:
Carrying capacity:
Exponential growth:
Logistical growth:
Density dependent factors:
Hypothesis:
Variables:
Groups:
Sketch of set up day one
Group Data table
Page 2 Class Data Table
Turn in:
Graph, Typed analysis questions, Conclusion
After grading, flip page over data table
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