Yeast Population Lab
Introduction:
What are yeasts?
Yeasts are unicellular organisms that belong to the kingdom fungi. One of the
characteristics of yeast is its’ ability to ferment (change) sugars into alcohol. The most
commercially significant yeasts are the related species: Saccharomyces cerevidiae. This
species is used to ferment the sugars of rice, wheat, barley, and corn to produce
alcoholic beverages and in the baking industry to expand, or raise, dough. This species is
commonly known as baker's yeast.
Yeasts reproduce asexually by budding off a copy of itself or sexually by creating spores
(tiny fungi seeds) The live in various habitats including: plant leaves, flowers, soil, salt
water, skin, and live symbiotically inside the intestines of warm blooded animals. Yeasts
can reproduce rapidly and can cause infections in animals and humans. Common yeast
infections include: diaper rash, thrush of the throat, and vaginal infections. The
eukaryotic cells of yeasts are used to study genetics because the cell processes are
similar to other eukaryotes including mammals.
The yeast's function in baking is to ferment (break down) sugars present in the flour or
dough. This fermentation gives off carbon dioxide and ethanol. The carbon dioxide is
trapped within tiny bubbles and results in the dough expanding, or rising. The
fermentation of wine is caused by naturally occurring yeasts present in the grape plant.
The bubbles in sparkling wines are trapped carbon dioxide, the result of yeast fermenting
sugars in the grape juice.
All species have a high reproductive potential (population growth). Insects
and fish, for example, typically lay several hundred eggs, while clams can release
several thousand eggs. Normally, this huge potential is kept in check by the carrying
capacity (resources available) of the environment.
During this laboratory session you will explore the rate of growth of a population of yeast
cells in a simple ecosystem (a test tube) and will study a mathematical model of population
growth. (Refer to notes on population growth)
Go to the link below for a visual of the asexual reproduction/budding in yeasts.
http://www.nformationdesign.com/content/port/cellcycle08.gif
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Purpose:
To study a population of yeast in a closed system over an extended period of
time.
Problem:
What is the effect of the amount of resources (food) over time on the growth of the yeast
population?
Materials:
Glucose solution (30g/500ml)
rehydrated yeast (3.5g/50ml)
2 small (10ml) test tubes
eye dropper
10ml pipette
Pipette bulb
slides and cover slips
iodine or methylene blue
masking tape
Day One
Procedure I: Culture preparation
1. Gather 2 small test tubes and label with your group name and letters A & B.
2. Add 5 mL of sugar solution to test tube A using the 10ml pipette and 5 mL of water to test
tube B.
3. Add 1 drop of yeast solution to each test tube with the eye dropper. Mix well by swirling the
beaker.
4. Test tube B with the water is your control and A with the sugar solution is your variable.
5. Place test tubes in test tube rack at front of classroom. The instructor will put the test tube
rack in a cabinet overnight.
6. Draw a labeled diagram (a picture!) of this procedure. SAVE this! It will be used in the
final lab report!
Day Two
Procedure I: 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. Make a wet mount slide by using a clean pipette to place one drop of yeast solution from test
tube A onto a slide.
***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!!!
3. Add a drop of Iodine to the yeast drop on the slide and cover it with a cover slip. Remove
excess water from around cover-slip edge with a piece of paper towel if needed.
4. Start at 4x and use coarse focus to get the cells in view. On medium power (10x) bring your
slide into focus using the fine focus knob.
5. Move to high power (40x we will not use 100x!) Count the number of yeast cells that are in
“contact” with your field of view only. Record the count in the data table.
6. Rinse your slide and cover slip and repeat steps 1 thru 5 for 2 more trials.
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7. Repeat steps 1 thru 6 for test tube B.
8. Return your test tubes 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)
Days 3+
Repeat procedure II from day two. Record all your data in your data table. Continue on writing the
lab report. At this time you can complete the Introduction and Procedure sections!
Data Collection:
Date:
Glucose Solution
Water Solution
Yeast Count #1
Yeast Count #2
Yeast Count #3
count average
Date:
Glucose Solution
Water Solution
Yeast Count #1
Yeast Count #2
Yeast Count #3
count average
Date:
Glucose Solution
Water Solution
Yeast Count #1
Yeast Count #2
Yeast Count #3
count average
Date:
Glucose Solution
Water Solution
Yeast Count #1
Yeast Count #2
Yeast Count #3
count average
Data Analysis:
After data is gathered you will create a line graph to display both sets of data (control and
variable). You should make one graph with 2 different colored and labeled lines. Once you’ve
complete the graph be sure to add a title and a sentence under that explains briefly what the graph
is about. For example: “This graph shows the growth of a yeast population…..”
Conclusion:
Follow your general lab rubric and notes on the scientific process to complete this section. Use the
following questions as a guide for writing your lab report paragraphs.
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. What would you expect to happen to the yeast population in the future? Would you expect the
number of yeast to change as they run out of resources? Did you see this in your data?
3. Why might our method of data collection be inaccurate?
4. Does your graph look like the graphs in our population notes? How?
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