YEAST POPULATION STUDY

INTRODUCTION

Yeast organisms are useful for studying populations due to their small size and ease of manipulation and culture. They have a rapid reproduction rate and their biotic potential is high. In this lab, you will observe yeast cells growing and reproducing in a sterile medium. This culture will take place in a limited, closed environment-that within the test tube. To the yeast, this

test tube is" earth ".

Objectives

Use sampling techniques to determine population density changes. Observe yeast population changes in a closed system. Calculate yeast population density based upon a dilution

series. Apply the principles discovered and observed in he yeast population to the human population.

Materials

tubes with caps .microscope .dropper .glass slides with cover slip

.cover slip fragments .pipette yeast culture other dilution equipment as required

flask or miscellaneous glass ware thermometer

Surveying the Population

After all of the tubes have been treated, the yeast population within must be counted. Yeast cells are microscopic, and some of your tubes will have lots of them, so to facilitate data collection you will use a technique known as sampling, where a small proportion of the population will be used to represent the entire population. If the population is too big to count effectively, you will dilute the population and then perform calculations along with your samples.

1. In order to prevent yeast cells from being crushed, place a few cover slip fragments on the slide in such a way as to support a full cover slip.

2. While covered shake the tubes and remove 1 drop of solution from it.

3. Place the drop on the microscope slide and put a cover slip over it.

4. Put the microscope on high power, and begin counting individual cells within the field of view. Buds count as individuals. If you see a cell with a bud coming off of it, the count would be "2". Yeast cells often flocculate, or stick together. Be sure to count individual cells, not clumps. If the field is too crowded to count, perform a dilution as previously discussed. Each dilution causes error to enter your preparation, so don't dilute unless necessary .Be sure to record dilution factors if dilutions were performed.

5. Move the slide to a new field, and re-count. Repeat to count 3 different fields from the same sample.

6. After 15-20 minutes have elapsed transfer 1-2 drops of your yeast culture to the slide. Repeat the counts using the sampling technique.

6. Continue to remove samples every 15-20 minutes, and count.

7. Repeat steps 1-6 , performing dilutions as required.