Yeast—On the Job
Student Laboratory Kit
Materials Included In Kit
Active dry yeast, 12 packages
Glucose, 50 g
Methylene blue solution, 10 mL
Cheesecloth, 1 square yard
Flour, 150 g
Glucose test strips, 100
pH test paper, 1 vial
Pipets, Beral-type, 48
Additional Materials Required
Water, distilled, 250 mL
Balance, 0.01-g precision
Flasks, 125-mL, 4
Microscope slides, 4
Rubber bands, 2
Weighing dishes or papers, 6
Cut the cheese cloth into 4" squares. This experiment requires a number of materials not provided in the kit (see the list of Additional Materials Needed). Be sure all materials are available for student use prior to starting the lab.
Methylene blue is a vital stain and will stain nearly anything including skin, clothing and table tops. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron during this activity. Please review current Safety Data Sheets for additional safety, handling and disposal information.
Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. All solutions can be flushed down the drain with volumes of water following Flinn Suggested Disposal Method #26b.
Student laboratory group size (for each set of four flasks) can vary from 1–4 students depending upon your class size, space, materials and goals. Two class periods are required to complete the laboratory. Enough test materials are supplied for twelve complete setups of the experiment.
- If it is possible for students to observe their flasks sometime between the initial setup class period and the 24-hour waiting period, the balloon reactions and observation patterns might be more dramatic.
- Do not dilute yeast solutions. The excess yeast culture assures consistent results in a 24-hour period.
- Many additional chemical combinations can be tested using the simple setup in this experiment. If time and equipment allow, encourage students to design additional experiments to illustrate factors that affect cellular processes in yeast cells.
- Provide a storage place where the flasks will be undisturbed for 24 hours and will be at least as warm as room temperature.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Analyzing and interpreting data
Planning and carrying out investigations
Disciplinary Core Ideas
MS-PS1.A: Structure and Properties of Matter
MS-PS1.B: Chemical Reactions
MS-LS1.A: Structure and Function
HS-PS1.A: Structure and Properties of Matter
HS-PS1.B: Chemical Reactions
Scale, proportion, and quantity
MS-PS1-2. Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
MS-LS1-1. Conduct an investigation to provide evidence that living things are made of cells; either one cell or many different numbers and types of cells
The following things will happen in a fairly predictable fashion:
1. The level of glucose will go down in flasks A and B. Very active bubbling and churning will take place as energy transformations are occurring.
2. The pH level in flask A will drop dramatically compared to the other three flasks and the balloon will swell, indicating the production of CO2.
3. Flask A will have the most distinctive odor on the second day. The distinct odor of alcohol will be easily detected, a further indication of cellular activity resulting in the synthesis of various chemical products.
Answers to Questions
Analysis of Observations
- Was there a change in glucose levels in any of the flasks after two days? If so, which one(s)? Explain why.
Flasks A and B have decreased levels of glucose after being consumed by the yeast.
- Was there a change in pH in any of the flasks after two days? If so, which one(s)? Explain why.
The pH in flask A decreased by the second day indicating the production of CO2.
- What happened in the flasks with sugar added compared to the flasks without sugar?
The flasks with sugar (A and B) showed the most chemical activity as evidenced by the bubbles and foam produced.
- What happened to the balloon on flask A? What happened to the balloon on flask C? Explain.
The balloon on flask A inflated with gas produced by the fermenting sugar. The balloon on flask C inflated very little if at all due to the lack of chemical activities.
- What did the contents of each flask smell like? Were any of them different from the others?
Flask A was different than the rest—it smelled like alcohol.
- Did the amount of yeast in any of the flasks increase? Which one(s)?
The amount of yeast increased in flasks A and B.
- Did any of the flasks produce a chemical that was not there before? Which one(s)?
Flask A produced alcohol. Flasks A and B both produced gas bubbles.
- Did any of the flasks use energy? Which one(s)?
Flasks A and B used energy.
Based on your responses to the analysis questions, answer the following questions about what cells do.
- What are four different activities or tasks that yeast cells perform?
The yeast cells maintained their internal environment to stay alive. They used sugar to produce energy. They produced a variety of chemical reactions. They reproduced to make more yeast cells.
- What evidence do you have that these activities or tasks took place?
The use of the glucose and the production of gas demonstrates the respiration and chemical processes of the yeast. Microscopic observations should reveal live cells with cellular movement and reproduction.
- If all cells perform the activities that yeast cells do (your answer to Question 1), then how might scientists define what makes something a cell?
A cell may be a unit that can:
a. Maintain itself in balance
b. Use energy (cellular respiration)
c. Carry out chemical reactions
- Since most living things are made of cells, use your answer to Question 3 to explain how a biologist might answer the question, “What makes something alive?”
Similarly, a living thing should exhibit the same characteristics listed in Question 3.
- Microscope examination of samples will show that active reproduction occurs in flask A and B. It is likely that, with careful microscopic observations, students will witness actual yeast budding occurring.
- The drop in glucose level, the production of alcohol, the production of CO2, and the cell division of the yeast cells all indicate the ability of the living cell to regulate what goes into and out of the cells. (The balance among changes—homeostasis.)
Currently, biologists recognize that there are four fundamental activities which nearly all cells engage in at some point in their life cycle. They maintain and regulate their internal chemical environments (homeostasis); transform energy for their own uses (cellular respiration); produce a variety of chemical products either directly (protein syntheses) or indirectly (enzyme governed chemical reaction); and reproduce at some point (though some may lose this ability in their life cycle). Together these four general processes help provide an initial understanding of a modern biochemical model of cellular life.
Most of us, however, have a hard time envisioning these abstract ideas. Students especially have difficulty. Giving students an opportunity to observe parts of these four processes actually occurring can be very contributory to developing a real understanding of the cell. This activity is intended to meet this exact goal. It can be used as an introduction to any unit on the cell or individual components can be adapted to study each specific cellular process by itself.
The basic procedure for the experiment is simple. In four different containers, students provide common bread yeast with four different environmental conditions. The first flask (A) contains a source of energy (glucose), elementary biochemicals (flour), and water, but is oxygen deprived by sealing it with a balloon. The second flask (B) contains the same ingredients, but it has a supply of oxygen. The third and fourth flasks (C and D) are deprived of energy, and one of them (flask C) is deprived of oxygen. Students leave the yeast under these conditions for 24 hours, testing glucose and pH levels before and after, and they observe the yeast for any other changes at both the macroscopic and microscopic levels. They then use a series of comparisons from these observations to deduce the four basic properties of life.
This kit was created by David Brock, Roland Park Country School, Baltimore, MD.
||Yeast on the Job—Student Laboratory Kit
||Water, Distilled, 4 L
||Flinn Scientific Electronic Balance, 410 x 0.01-g
||Beakers, Polymethylpentene (PMP), 250 mL
||Cover Slips, Plastic, Square, 22 mm x 22 mm, Pkg. of 100
||Microscope Slides, Glass, Best Quality
||Rubber Bands, Medium, Pkg. of 750
||Weighing Dishes, Disposable, 3-1/16" x 3-1/16" x 1", Pkg. of 500
||Weighing Paper, Moisture-Resistant, Non-Absorbant, 6" x 6", Pkg. of 500