Yeast Population Study: This experiment
will grow on you!
Perhaps you’ve heard of yeast. They are one-celled organisms, widespread in nature, living
in soil and on plants. Invisible to the naked eye, these fungi are important to humans for
their ability to ferment carbohydrates. Beer, wine and bread making exploit the yeasts’
break-down of sugars (sugar is their food source); alcohol
and carbon dioxide are by-products. Most cultivated
yeasts belong to the genus Saccharomyces; those known
as brewer's yeasts are strains of S. cerevisiae. Yeast are
also used medicinally, as a source of B-complex vitamins.
You can buy yeast at the grocery store, dehydrated and
clumped together. On the down side, yeast can also
cause infections in animals.
In this lab, we will grow these microbes to observe
changes in their population over time. Given that yeast
are microscopic fungus, little objection is raised to their
use in experimentation. They grow fast, require little
care and illustrate principles of population growth,
nicely. The picture above shows magnified yeast cells. The “pimples” on the cells are
buds. Yeast reproduce asexually by budding; they do reproduce sexually, as well.
Population biology
In any population of organisms, two categories of factors control population size and
growth, biotic potential and environmental resistance. Biotic potential is all the "right"
conditions for population success such as optimum light, plenty of food, ideal temperature,
and enough water. Environmental resistance refers to conditions that limit population
growth such as inadequate light, scarce food, extreme temperatures, and drought. The
condition that most limits growth is the limiting factor: the one condition that will provide
the "cap" to population size or define the carrying capacity (K) of a population. Sometimes
limiting factors are long-lasting; however, evolution and the dynamic nature of the
environment often promote a shift in limiting factors thus rarely is a carrying capacity ever
exactly the same number in a population for an extended period of time.
In order to experience sustainable growth, that
which does not overly tax an environment or its
resources, most populations will experience a short
period of J curve or exponential growth, an
unsustainable period where biotic potential exceeds
environmental resistance, before lapsing into S
curve or logistic growth about a carrying capacity.
Observe the graph of the white-tailed deer
population here.
During the exponential period of growth, some
populations have growth rates that can be calculated
mathematically using a natural log (ln) or e
formula. In environmental science, we are often concerned with the point at which the
population will double. This time is identified as DT for doubling time and defined based
on r, the population growth rate. The natural log equation for doubling time is:
DT = ln (2) / r
where r is the population growth rate in DECIMAL FORM. Because the natural log of 2 is
approximately 0.70, our simplified no calculator formula for doubling time is the following:
DT = 70 / R
where R is the population growth rate in PERCENTAGE FORM.
This week we will study the population growth rate of yeast, Saccharomyces cerevesiae.
Given plenty of food, they should grow happily for a while. We will watch them over a
week; what do you think will happen as the week proceeds? Why?
Procedure
If your yeast cultures become contaminated your experiment will be ruined. So start by
cleaning your lab table with Windex. Wash your hands with soap. Use only glassware that
has been sterilized.
1. Spread paper toweling on your work area. Work on the paper toweling.
2. Obtain 5 test tubes and place them on a test tube rack.
3. At the center table, carefully pour out 20 ml of nutrient solution into a clean, 100 ml
beaker. Bring this to your table.
4. At the center table, stir the yeast culture. Carefully pour out 20 ml of yeast culture into
another clean, 100 ml beaker. Bring this to your table.
5. At your table, using a pipet and pipet pump, measure 3 ml of nutrient solution into each
test tube. Alternatively, you could use graduated plastic pipets, if available.
6. Next, carefully add 3 ml of yeast to each test tube using a clean pipet and pipet pump.
Make sure to stir the beaker so your yeast culture is evenly distributed.
7. Cover each test tube tightly with parafilm. Stretch the film over the mouth of each tube
and seal. Shake the tubes hard for ~30 seconds each. Open the parafilm to allow gas to
escape. Reseal the parafilm on each tube.
8. Clean up all unneeded glassware and return it to the cart provided.
Surveying your population
Day 1-4, you will sample your yeast population and count cells. We will leave the cultures
to grow over the weekend and do a final sampling on Monday. Follow this procedure each
day, USING A NEW TEST TUBE each day.
A. Open the parafilm on the “day 1” tube and add 5 drops of methylene blue solution.
Be careful! MB stains.
MB is a chemical that is chemically changed by living cells. The yeast cells will
change from blue to clear if they are alive. That means blue cells are dead!
B. Replace the parafilm tightly and invert the test tube, gently, 5 times, to mix
everything.
C. Get 2 microscopes, 2 slides and 2 coverslips.
D. 2 of the members of your group should each remove a small drop of culture from the
middle of the tube. Place the drops on separate microscope slides and carefully add
a cover slip to each. Don’t crush the yeast! You should now have two samples from
the tube.
E. Each pair of two people should focus the yeast cells under their microscope, using
low power (100x). Switch to the next higher power (440x). Ask if you need help
with the microscope!
F. Next, you want to count the yeast cells in your field of vision. If a cell is budding,
count it as two. If there are too many to count, mentally break the field of vision
into ½ or even ¼ if necessary. Count all the living cells (the clear ones). Multiply by
2 or 4, as necessary. Record your counts in the table provided. Move your slide to a
new field and recount as time permits.
G. Repeat a-f Wed, Thurs, Fri and the following Monday. Make sure to record all data.
Data
Day (use a new
tube each day)
Tube one
First field of
view, # of cells
Pair 1
Pair 2
Tube two
Pair 1
Pair 2
Tube three
Pair 1
Pair 2
Tube four
Pair 1
Pair 2
Tube five
Pair 1
Pair 2
Second field of
view, # of cells
Average number
of live cells
Class data
Data Analysis:
After completion, you will create a line graph (If using Excel it’s a scatterplot) to
display both sets of data (your group and class). You should make one graph with 2
lines; one for your own average yeast population over the course of the experiment
and a line for the class average yeast population over the same period of time. Title
your graph and label the axes, as you should do on all graphs. Include a key. 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. Describe the population growth throughout the experiment. Why does the yeast
population change in such a manner?
3. What environmental resistance did the yeast face? I.e. what limited their growth?
4. Determine the doubling time of the yeast population in your initial J curve. Use the
doubling time equation to find the R.
5. What would you expect to happen if we let the yeast grow another few days? Another
week? Another two weeks? Explain.
6. Do you think your group data are more accurate, or are the class data better? Explain.
7. Why might our method of data collection be inaccurate? What problems did you have as
you collected data in this experiment? Do you think your/our data are reliable? Make
sure to mention sampling error.
8. How does human population growth compare to yeast population growth? What do you
think would be the same? What would be different? Are there limits to growth in a test
tube? How about on planet earth? Explain.
Your group will turn in one report including graph(s) and complete answers to the above
questions. 20 points.