Observing a Yeast Population
Yeast are single-celled microorganisms that reproduce
by budding. They are fungi, are not capable of
photosynthesis, therefore, convert sugars into other
chemicals like ethanol as a waste product in the
absence of air (anaerobic), and produce carbon dioxide
when air is present (aerobic).
Today your group is going to be given a quantity of dry
yeast. They purpose of this lab is to determine if you
can keep the population of yeast growing for five days. You will also explore the factors
necessary to keep the yeast alive and dividing. Finally, you will determine what resources limit
population growth.
Materials
package of yeast
syrup
sugar
distilled water
warm water baths between 20oC and 45oC
beaker
25mL graduated cylinder
stirring rod
thermometers
3 test tubes
beral pipet
microscope
microscope slides
cover slips
Procedure
1. Determine which variable your group will be responsible for, either, the amount of food,
temperature or amount of yeast. Decide on control factors before you carry out your
experiment, that is, make this a fair test. If you are testing the how population changes
with different amounts of food, you need to know exactly how much food each sample of
yeast is given, among other things.
2. Add a quarter teaspoon of yeast to the beaker then stir in 30 mL of distilled water. Add
10 mL of the yeast mixture to each test tube. (You will need to decide how much yeast
mixture to add to each test tube if you choose the amount of yeast as your variable.) Add
about 4 mL of syrup to each test tube, or add sugar to each tube, and decide how much
syrup to add to each test tube if you are using the amount of food as a variable. Store your
yeast mixtures in labeled test tubes in the beaker that you have rinsed at room
temperature, unless you have chosen to vary the temperature, in which case you will need
to label each tube with your name and #1, 2 or 3, and place them in warm water baths of
different temperatures. Cover the test tubes with plugs of cotton.
3. Record your plans in your lab report.
4. If there is time, take a sample of the yeast population by first stirring the yeast mixture,
then using the beral pipet, remove a small portion of the mixture from about the middle
of the test tube and place one drop on a microscope slide, and cover the drop with a cover
slip.
5. View the drop in the microscope on medium power. Count the number of yeast cells in
contact with the outside circle of your field of view. Record the test tube label and
number of yeast cells touching the outside of the field of view. Follow your teacher’s
instructions about the microscopes and where to place the used microscope slide and
cover slip.
6. Repeat steps 4 and 5 with the other two tubes.
7. Continue steps 4 to 6 on each of the consecutive days for the rest of the week, being
careful to record your results.
Data
The following tables may help you organize your data.
Tube #
1
2
3
Tube #
1
2
3
Description of how tubes differ from each other
Day 1
Day 2
Day 3
Day 4
Analysis:
1. Describe how the population changed in each tube over the period of the lab
investigation.
2. Use an Excel spreadsheet to make a table of the data and graph the results.
3. Which populations were most successful, and which were least successful? Talk to other
groups who used the same variable about their results.
4. Did you find evidence of a limiting resource in your investigation? What was it? How do
you know it was a limiting resource?
5. Plan a one minute presentation summarizing your investigation to the class.
Conclusion:
Describe the results of your lab. Include what factors were held constant, and which factor was
the variable. How did the variable affect the growth of the yeast population?