Yeast Inquiry Part 1:
Quantitation of Cells
BACKGROUND: Baker’s yeast (Saccharomyces cerevisiae), has
been used by scientists as a “model organism” for decades. In fact,
this single-cell fungus is the most studied eukaryotic cell in the
history of biology! Our earliest understanding of cancer came
from studies on how yeast cells divide. This was the first
eukaryotic cell to have its entire genome (all of the genes in its
DNA) sequenced. When scientists look for a useful model
organism, they look for several features:
1. size
2. ease of manipulation
3. generation time
4. cost
Yeast cells are inexpensive and easy to grow. Because this
microbe is a eukaryote (ie contains a nucleus and organelles), it
has many similarities to our own cells. For these reasons,
scientists love to study yeast.
OBJECTIVE: For this first step in our inquiry into the properties
of yeast, you will learn how to use a microscope and prepare slides
for viewing. You will use several key pieces of equipment and
glassware, including a measuring pipette and graduated cylinder.
You will learn how to prepare dilutions of cells in order to count
them. It is important to read all steps in your lab exercise.
Safety tip: Handle glass slides and cover slips with care. These
are extremely fragile! Carry microscopes appropriately, by the
arm, with one hand always supporting under the base (see photo).
ANATOMY of a COMPOUND MICROSCOPE
Familiarize yourself with the working parts of the compound
microscope. The relevant parts of the microscope are the eyepiece,
the fixed stage on which to place your specimen (in this case a
slide with a drop of yeast suspension), several objectives with
different magnifications and a condenser that focuses light from
underneath onto your specimen.
MATERIALS:
You will work in groups of 2-3. You will learn the working parts
of the microscope as well as how to measure using a graduated
cylinder and a graduated pipette. You will prepare serial
dilutions of a yeast suspension (yeast in water with no sugar)
prepared by Dr.B.
YEAST
To make dilutions:
 Your test tubes are labeled 1/10, 1/100 and 1/1000
respectively.
 Have one person take the beaker to the sink and get 40-50
mLs of water from the tap.
 Using the graduated cylinder, put 9 mLs of water into each
of the three test tubes. When measuring, be sure to pay
attention to the MENISCUS (See picture). You read the
volume by looking at the line that matches the lowest part of
the meniscus.
meniscus

 Have a person take the 1/10 test tube to the back of the lab
and get 1 mL of yeast suspension from the flask prepared by
Dr. B. Add the 1 mL of suspension directly into your tube
and mix well (by pipetting up and down).
 Make the remainder of the dilutions according to the
following table:
Table 1: Dilution Preparation
Dilution
1/10
1/100
1/1000
Yeast
1 ml of yeast
suspension
1 ml of the 1/10
dilution
1 ml of the 1/100
dilution
Water
9 mls of water
9 mls of water
9 mls of water
 Now add 1 drop of methylene blue to your three
dilution tubes and mix well. This will help us to
visualize our yeast cells.
 Place a drop of yeast dilution on a slide and cover with
a cover slip (see figure).
 Focus on the yeast with low power and then move up to the
high power objectives. Using the 40X objective, count all the
yeast cells in a view field and record in the table below:
Dilution
Number of
Yeast
Number of
Yeast (other
table group1)
Number of
Yeast (other
table group 2)
1/10
1/100
1/1000
Look at your results in the table. What do you notice about the
numbers?
Compare your results with those from 2 other table groups. Are
your results similar or different? If your results were different,
why do you think this could be?
What can we do with your results now?
 To determine the number of cells of yeast present, multiply
the observed count by the dilution factor.
Extending our inquiry
Now that you know how to visualize yeast using a microscope, you
can plan an investigation that will allow you to manipulate how a
yeast community responds to EXTERNAL factors. Think about
how you could use the equipment you have used today to test the
effects of different variables.