1. Investigating population growth
Yeast Explosion Lab
Investigating population growth

2. Background Yeast: Powdered & unicellular
Yeast are small microorganisms that reproduce rapidly. We will be using a common baker’s yeast to observe a population in action. The yeast will be grown in a test tube, so it will be a closed population (which will make it easier to measure the population growth because no organisms can enter or leave).
All living things carry on the process of cellular respiration. Respiration that occurs in the presence of glucose is called aerobic while respiration that occurs without oxygen is anaerobic. During respiration food, usually in the form of glucose, is broken down by a series of chemical reactions. As the glucose is broken down the cells transfer its energy to a molecule called ATP, or adenosine triphosphate. Cells use ATP as energy for the numerous chemical reactions that occur in the cell.
Respiration rate depends on several environmental factors. In this laboratory activity one of these factors, temperature, will be investigated in yeast cells. Yeasts are eukaryotic, unicellular fungi that are commercially important. They are necessary for the production of bread, wine, beer, and industrial chemicals. As the cells carry on cellular respiration they will break down glucose and release carbon dioxide and water. The rate of respiration in yeast will be measured by counting the number of carbon dioxide bubbles released.
Teacher note: The study of ecology typically involves populations, communities & ecosystems, and some form of human impact/restoration. As a start to this unit therefore, I want students to again investigate a process in a hands-on way, practice numeracy, uncover patterns of population growth, and make arguments with evidence. If done well, this will set them up to broaden their understanding to later ecology topics. In particular, looking at abiotic and biotic factors that influence population growth in areas like Isle Royale, a nature preserve in the Great Lakes area and the interplay among wolf populations, moose populations, and their mutual interactions will have been introduced by this activity.
Yeast: Powdered & unicellular

3. Overview of Experimental Design: Counting Yeast
The challenge of counting yeast, a microscopic organism, is two-fold: obviously they are incredibly small and their population levels can change rapidly therefore keeping track of them is quite difficult. If grown in a test tube, one might reasonably take a diluted sample, prepare a wet-mount slide of it, and count them using a grid system like the one shown above. Of course, this involves estimation due to the fact that a slide represents multiple fields-of-views and the sample is a dilute sample. Students find this to become tedious quite quickly and, unless there are multiple counts to verify population numbers, errors can easily mount.

4. Overview of Experimental Design: Suggested Method
Analyzing the Set-Up
Test tube
Water
Plastic dropper
Metal Nut
Yeast
Glucose (that produces CO2 bubbles via cell respiration)
Environmental condition
In order to activate yeast from powdered form, refer to this video:
This modified apparatus takes a simpler approach to counting yeast cells. A key assumption is that, since yeast perform cellular respiration which in turn creates carbon dioxide bubbles, more yeast will make more bubbles. Conversely, fewer yeast cells will make fewer bubbles. Of course, this is a simplified set-up but it does a reasonable job.
Special note: I use 15 x 150 mm test tubes and 1 mL plastic pipettes (with tips cut down to remain fully immersed in the water bath). The metal nut can be substituted with anything that keeps the pipette at the bottom of the pipette due to buoyancy that is created by the gas.

5. Hypothesis (Prediction)
Various questions could be posed regarding the relationship between environmental conditions and their impact on a single organism (Yeast).
For instance, does temperature increase the rate of reproduction and consequently the population size?
As a team, create both a question and a hypothesis that you will investigate in this experiment. Be sure to follow proper formatting:
“If…(MV) then…( RV) because…(prediction reason)”

6. Lab Details Question Materials Manipulated Variable
Responding Variable
Hypothesis
ECC
CVs
Since this lab happens later in the school year, the necessary scaffolding (designing experiments and the details involved in the process) has happened and students ought to be able to complete the remaining design on their own.
I use a five minute measurement window each class period for three to four days as students measure the effect of their chosen variable (temperature, %sugar solution, etc.).

7. Remaining Team Tasks Levels Repeated Trials
Procedure (I will most closely examine this section)
Data Table & graph
Conclusion
Since this lab happens later in the school year, the necessary scaffolding (designing experiments and the details involved in the process) has happened and students ought to be able to complete the remaining design on their own.