www.TeachEngineering.org: Free Curriculum for K-12

See the Lesson Background and Concepts for Teacher in Lesson 1 (What Do Bread and Beer Have in Common?). Additional information that may be helpful is the fact that chemical reactions in general occur more quickly at higher temperatures than they do at lower temperatures. The chemical reactions involved in respiration are no exception. Since yeasts do not control their internal temperatures, all their metabolic reactions must occur at whatever the ambient temperature is. Therefore, yeast population growth should occur more slowly at cooler temperatures than at warmer temperatures.

The yeast species used in both bread baking and the brewing of ales, Saccharomyces cerevisiae, thrives at temperatures between about 5º C and 55º C (about 40º F to 130º F). Ale makers prefer a relatively slow fermentation process to produce the best taste, so they typically use temperatures around 17-22º C (62º-72º F) for fermentation. This process can take days or weeks before the desired alcohol content is reached. Bread bakers require a much faster process, so they prefer leaving the bread dough to rise in a warmer place, at least 27º C (80º F), but not above 49º C (120º F). Temperatures that are too warm make the bread rise too quickly, resulting in large holes within the finished product.

Tell students they will be working in groups of four to design an experiment that will test for an environmental condition they think would be either favorable or unfavorable for yeast population growth. Tell them that in order to design their experiments, they must have answers to the following questions:

What is the specific question you are asking?
How exactly will you try to answer it?
How many trials will you do?
How will you report your results quantitatively?
What will be your control(s)?
What is your hypothesis?

Before having them start on their own designs, discuss how these questions would have been answered for the initial experiment done in the Yeast Cells Respire, Too (But Not Like Me and You) activity. In that case, the answers to the questions above are:

"Can we obtain evidence that in the process of respiration, yeast cells convert the sugar in molasses to carbon dioxide and alcohol?"
Students should be able to describe exactly how the experiment was set up. Important aspects of the design were that the chamber included the elements known to be needed for yeast respiration: a supply of glucose and a wet environment, as well as a way to observe and measure the carbon dioxide given off by respiring yeasts.
The number of trials done was equal to the number of test chambers set up, since all were set up the same way and exposed to the same conditions. Point out that it is important to have several identical trials in an experiment. Three trials is generally accepted as the minimum number. Multiple trials are needed for several reasons.

First, a mistake could be made in preparing one (or more) of the test chambers, which could produce a misleading result. For example, if only one chamber was set up and observed in an experiment, and the experimenter forgot to add the yeast, the experimenter would have concluded that it was impossible to observe yeast respiration, since no gas bubble was observed. Or it might also be possible that the experimenter did put yeast in the chamber, but a contaminant was somehow present, which poisoned the yeast and resulted in no respiration. On the other hand, if several chambers had been set up, and only one produced no bubble or a very small bubble, the experimenter might suspect that something was wrong with that one chamber.

Furthermore, organisms and biological systems in general exhibit a great deal of variability in anything they do. A biologist would not expect to see gas bubbles of the exact same size in each test tube even if multiple trials were conducted. Instead, variable bubble sizes would be expected because of the natural variation within the yeasts themselves, and also because of the design of the experiment. There was no way to be sure that an equal number of yeast cells was added to each test tube, nor was the volume of molasses solution exactly the same in each tube. Therefore it is reasonable to expect slightly different sizes of gas bubbles to occur in the different test chambers. However, with multiple trials, the experiment did allow the original question to be answered. It also provided additional information, such as how long, on average, it took to produce bubbles of various sizes.

The results were reported in the form of the graphs students prepared, which showed how the sizes of the gas bubbles changed over time.
There was no control for the initial experiment. However, it is very important to have one! The control that should have been used would be at least one test chamber set up and treated exactly like the rest but containing no yeast cells. The reason a control was needed was because without it, how could we know that the gas bubble collected did not simply come from a chemical reaction within the molasses? It is possible that the yeast did not respire at all, and what we observed was the product of some other reaction occurring in the solution. If a few control chambers had been used, and no bubbles formed in them, then we could state with confidence that the yeast cells in the test chambers were responsible for the gas bubbles instead of the molasses alone.
The hypothesis was that the inverted test tube would become filled with a visible gas bubble due to the yeast respiration.

Following this discussion of the design of the previous experiment, divide the class into groups of four. Ask each group to decide on an environmental factor to test, and give them time to discuss their answers to the list of questions. Tell them they will need to prepare written proposals (one per group) for their experiments, but they should first check out their ideas with you orally.

If students want to see if different amounts of food affect population growth, encourage them to consider a logical sequence of molasses concentrations, such as 0%, 25%, 50%, 75%, and 100%. Have the students calculate for themselves how much water and how much molasses need to be combined in order to obtain the desired concentration in 250 ml of each solution. Remind them also that they need multiple trials for their experiment. In this case there should be three or four chambers set up at each concentration to be tested. If two or three student groups want to explore the effects of food supply; they can be combined into one large group to help with the problem of adequate sample sizes.

Likewise, student groups investigating other environmental factors may need to be combined in order to achieve adequate sample sizes if supplies are limited.

Be sure that each group includes controls in its design. A group testing molasses concentrations, for example, should include a chamber set up for each concentration that does not contain yeast. The same is true if students test two or three different temperature conditions: at each temperature there should be at least one chamber without yeast. If students test different pH levels of the molasses solution, they will also need a yeast-free control for each pH level tested.

Be sure that students realize they can only test one environmental variable at a time. If they propose to test different concentrations of molasses at different temperatures, for example, they will not be able to identify which of the two factors is responsible for the results they obtain. Point out that if they want to find out how both temperature and food supply affect population growth, they will have to conduct two different experiments. In this case it would be more efficient to have half the class test molasses concentration, while the other half tests temperature differences.

If students wish to test the effects of pH, encourage them to use a 20% molasses solution for their experiment. That way, if they need to add more than a few milliliters of lemon juice or vinegar to acidify some of their test chambers, it will have a relatively slight dilution effect on the molasses solution. After determining the pH of the 20% solution (which they should make up themselves), they should try to achieve pH changes of only 1 and 2 in either direction, so as to not overly dilute the molasses. (Sodium carbonate can be added directly to the molasses solution; it does not need to be mixed with water first.)