YEAST POPULATION LAB

The growth of populations and the maximum population size in a habitat are affected by the availability of nutrients, physical conditions in the habitat, and biotic interactions. In this lab we will follow the growth of yeast populations in a habitat with differing conditions. Food concentrations for the yeast will be varied by changing the amount or type of sugar in the growth medium. We will attempt to assess the effect of varying sugar concentration or type on the percent growth of the yeast population (PG %) / day.

Direct counts of the yeast will be impossible, so we will estimate population size with a spectrophotometer. A spectrophotometer is a machine which shines light through a sample to measure its density. The greater the density (and hence number) of cells in the sample the higher amount of light absorbed causing a higher absorbance reading on the spectrophotometer. Each group will monitor the growth of their cultures over five days and the cultures will be kept incubated at room temperature.

SETUP PROCEDURE (per group):

1.  Obtain approval of your experimental materials and procedure from your instructor. Don’t forget to include a control tube!

2.  All tubes used in this lab contain 25 ml of sterile Sabouraud Dextrose broth.

3.  5 drops of 1% yeast suspension should be added to each tube used in the experiment.

4.  The amount and type of sugar used, or type in the tubes will vary from group to group.

MEASUREMENT PROCEDURE:

1. Your instructor will make sure the spectrophotometer is set to read the absorbency for each tube. In order to do this a control tube (Sabouraud Dextrose Only) is inserted into the spectrophotometer and calibrated so that the absorbance reads 0 on the spectrophotometer.

2. Immediately before obtaining a reading on each tube, finger vortex the tube so that spinning column of medium reaches the bottom of the tube for several seconds. This is critical! The yeast cells are heavy and will tend to sink to the bottom of the tube, so you must finger vortex the tubes to resuspend them: otherwise, your spec readings will be falsely low.

3. Using a pipet obtain a small sample of one tube and fill a cuvette 2/3 full of medium. Bring the cuvette to your instructor to obtain an absorbance value for that tube. All members of your group should record this data as Day 0 in a notebook. Pour the medium out of the cuvette back into the tube you obtained it from.

4. Repeat procedures number 2 and 3 above for each of the tubes in your experiment each day rinsing the pipet and cuvette between each measurement.

5. Research the questions given to you on the Yeast Population Rubric in order for you to come up with a hypothesis based on sound background research for your experiment.

6. Keep in mind that in order for the results of your experiment to have any significance or meaning, take steps to ensure that controlled variables are kept to a minimum and great care is taken during the experimental set up and data collection each day.

7. Our hope is that this activity will be of some interest to you and help you better understand how the scientific method can be used in a high school lab setting.