Materials Included In Kit

Additional Materials Required

Prelab Preparation

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.

Safety Precautions

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.

Disposal

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.

Teacher Tips

• 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.

Sample Data

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

1. 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.
   

2. 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.

3. 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.

4. 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.

5. 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.

6. Did the amount of yeast in any of the flasks increase? Which one(s)?
   

   The amount of yeast increased in flasks A and B.
   

7. 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.
   

8. Did any of the flasks use energy? Which one(s)?
   

   Flasks A and B used energy.
   

1. 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.
   

2. 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.
   

3. 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
   d. Reproduce
4. 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.

5. 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.
6. 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.)

Discussion

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.

Teacher Handouts

10255_Teacher1.pdf

References

This kit was created by David Brock, Roland Park Country School, Baltimore, MD.

Introduction

The most fundamental question in biology—what makes something alive? Observing the yeast cells in this activity will provide an initial set of data for formulating an answer to this question.

Concepts

• Cellular respiration

• Chemical degradation
• Properties of life
• Mitosis
• Chemical synthesis
• Homeostasis

Materials

Safety Precautions

Methylene blue is a vital stain and will stain nearly anything including skin, clothing and tabletops. This activity requires the use of hazardous components and/or has the potential for hazardous reactions. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron during this activity. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

1. Yeast cells are very hardy organisms, but they do need special conditions to grow. It is important that the directions for setting up this laboratory are followed carefully. Label all containers, observe carefully, and keep accurate and detailed records.
2. Using a balance and weighing dishes, measure two separate 1.25 g amounts of glucose and set them aside. Then measure four separate 2.5 g amounts of flour and set them aside.
3. Place 125 mL of very warm distilled or deionized water (approximately 46 °C) in a 250-mL beaker and add one package of dry, active yeast. Allow the yeast to sit for a few minutes and then gently swirl the solution until the yeast dissolves.
4. While the yeast is further dissolving, label four flasks “A,” “B,” “C” and “D,” respectively.
5. Place 2.5 g of flour in each of the four flasks. Carefully add 30 mL of the yeast mixture to each of the four flasks. Gently swirl the contents of each flask. Examine the flasks and record what you see in each flask on the Yeast—On the Job Worksheet (e.g., color, bubbles, turbulence, odor).
6. Next add 40 mL of distilled water to each flask and then immediately add 1.25 g of glucose to flasks A and B. Gently swirl all four flasks to mix.
7. Test all four flasks for the presence of glucose and determine the pH of the solution in each flask. Complete these tests by dipping a piece of appropriate test paper into the liquid contents of each flask. Use the color comparators for the readings and use a separate test strip for each flask. Record the results on the Yeast—On the Job Worksheet.
8. Secure an uninflated balloon and carefully place the opening of the balloon over the opening of flask A. Make sure the balloon securely seals the opening of Flask A. Use another balloon and repeat this procedure for Flask C.
9. Place two small pieces of cheesecloth over the openings of Flasks B and D. Use a rubber band to secure the cheesecloth in place.
10. Observe the flasks throughout the remainder of the laboratory period. Record your observations on the worksheet, especially noting any activity within each flask.
11. Place the flasks in a warm place for storage overnight.
12. On the second day observe all four flasks again. Look carefully for any changes and record your observations on the worksheet.
13. Remove the balloon from flask A and carefully smell the air coming out of the balloon and the top of the flask. Do this in a wafting fashion without taking a deep breath. Record these “nose observations.” Repeat this smelling procedure for flasks B, C and D.
14. Use test strips to test the liquid contents of each of the flasks for the presence of glucose and for pH. Record the results.
15. Label four clean microscope slides A–D, respectively. Using a clean Beral-type pipet for each flask, collect a sample of the liquid from each flask and place one drop in the center of each slide. Place a coverslip over each sample drop.
16. Examine each slide under a microscope at 40X power and record your observations. Examine the yeast carefully and take more samples if necessary. Use the back of the worksheet to make drawings of your yeast cell observations.
17. Repeat steps 15–16 adding a drop or two of methylene blue stain solution to each sample before adding the coverslip to each slide. Record all observations.
18. Answer the questions in the Analysis of Observations and Conclusions sections of the Yeast—On the Job Worksheet.

Student Worksheet PDF

10255_Student1.pdf