Yeast Population Study Lab

In order to study populations, it is important to be able to conduct investigations and make estimations of the population using sampling techniques. In this investigation you will study a population of yeast in a closed system over a nine day period. The investigation should be started so that counts begin a Thursday or Friday. This will avoid the problem of peak population days falling on a weekend.

If you do not have counting eyepieces for your microscope, use a transparency of a sheet of small-scale graph paper. Remove the eyepiece from the microscope, place if right side up on the transparency and outline it with a fine tip marker. Using sharp scissors, cut out a circle that is slightly smaller than the diameter of the circle you have drawn. Insert this grid into the eyepiece.

Materials

Procedure I: Culture preparation

1. Bring the sugar solution to a boil to sterilize it.

2. Cover and allow the sugar solution to cool to room temperature.

3. Boil some water to use as a sterile water bath for cooling your flamed inoculating loop.

4. Place a few grains of dry yeast into warm water in a small beaker and mix well to form a suspension.

5. Add the yeast solution to the sugar solution. Mix well.

6. For each trial, pour about 15 mL of yeast/sugar solution mixture into a test tube and cover with aluminum foil.

Procedure II: Sampling

1. Swirl the contents of the test tube to distribute the yeast throughout the liquid.

2. Flame and cool the inoculating loop and use it to transfer a drop of culture to the microscope slide. Prepare a wet mount.

3. Examine the slide under low and medium power to find a convenient magnification for viewing and counting. Make sure you use the same magnification each time you count.

4. Count the number of yeast cells in five squares within the field of view. Calculate the number of yeast cells per square.

5. Determine the area of the cover slip in "squares." Multiply the average number of cells per square by the number of squares in the cover slip. The amount of liquid under the cover slip is about 0.1 mL so you can multiply your answer by 10 to find the number of cells per mL. Record the result.

6. Repeat the count at the same time each day for 10 days. Record all your data.

7. Draw a graph of the yeast population versus time.

Questions

1. Describe the shape of the graph

2. Explain the population growth between days 1 and 6.

3. Explain the population growth from day 7 on.