Students set up a simple way to indirectly observe and quantify the amount of respiration occurring in yeast-molasses cultures. Each student adds a small amount of baking yeast to a test tube filled with diluted molasses. Then a second, smaller test tube is placed upside-down inside the solution. As the yeast cells respire, the carbon dioxide they produce is trapped inside the inverted test tube, producing a growing bubble of gas that is easily observed and measured. Students are presented with the procedure for designing an effective experiment; they learn to think critically about experimental results and indirect observations of experimental events.
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- International Technology and Engineering Educators Association: Technology
- I. Chemical technologies are used to modify or alter chemical substances. (Grades 6 - 8)  ...show
- H. Biotechnology applies the principles of biology to create commercial products or processes. (Grades 6 - 8)  ...show
- L. Biotechnology has applications in such areas as agriculture, pharmaceuticals, food and beverages, medicine, energy, the environment, and genetic engineering. (Grades 9 - 12)  ...show
- Next Generation Science Standards: Science
- Develop a model to describe how food is rearranged through chemical reactions forming new molecules that support growth and/or release energy as this matter moves through an organism. (Grades 6 - 8)  ...show
- North Carolina: Math
- North Carolina: Science
- Understand the processes, structures and functions of living organisms that enable them to survive, reproduce and carry out the basic functions of life. (Grade 7)  ...show
- Understand the composition of various substances as it relates to their ability to serve as a source of energy and building materials for growth and repair of organisms. (Grade 8)  ...show
- Analyze the relationships between biochemical processes and energy use in the cell. (Grades 9 - 12)  ...show
- Analyze photosynthesis and cellular respiration in terms of how energy is stored, released, and transferred within and between these systems. (Grades 9 - 12)  ...show
- Compare and contrast cellular respiration in yeast vs. plant and animal cells.
- Describe the role of yeasts in the production of bread and alcoholic beverages.
- large test tubes, about 15 cm long and 20 mm in diameter; one per student
- small test tubes, about 10 cm long and 8 mm in diameter; one per student
- squares cut from plastic wrap, about 8 cm on a side; one per student
- several rubber or cork stoppers, size 2
- test tube racks to hold large test tubes
- several dropping pipettes
- five 300-ml beakers
- 1-liter flask
- 1-liter graduated cylinder
- lab thermometer
- package dry baking yeast (available in grocery stores)
- 12-ounce bottle molasses (unsulphured)
- On the first day of the activity, students set up the yeast-molasses respiration chambers. It is best to start this activity on a Monday, so it can be monitored throughout the week.
- For the next 3-4 days, students take a few minutes of class time to observe their chambers and measure the heights of the carbon-dioxide bubbles within.
- On the last day, when most of the bubbles are at the maximum measurable height, students make graphs that show how the heights of the gas bubbles changed over the duration of the experiment.
Before the Activity
- Make sure test tubes, stoppers, pipettes and beakers are clean. If at all possible, fill a large pot with water, put all the glassware in, and boil for 10 minutes; then allow them to drain and air dry. This can be done up to several days in advance, as long as the clean, dry materials are kept in a clean, sealed container until ready for use.
- Prepare 1 liter of 10% molasses solution in the flask, by combining 100 ml of molasses with 900 ml of tap water and swirling gently for several minutes.
- Divide this mixture among four of the 300-ml beakers. Students will use these to prepare their "yeast respiration chambers."
- Just before class starts, use the remaining 300-ml beaker to prepare the yeast solution. Put about 200 ml of warm water (about 43-48 ºC) in the beaker and stir in 1 teaspoon yeast. Keep this mixture warm until students are ready to use it. Placing the beaker in a slightly larger container filled with water that is about 5 ºC warmer than the yeast solution works well. Check the temperature of the yeast solution every 5-10 minutes, though, and replace the surrounding water with warmer water as necessary.
- Practice demonstrating how to set up the yeast respiration chambers; this is best done while working over a sink or basin:
- Fill one of the large test tubes with water to within about 3 cm from the top. Then fill one of the small test tubes by pouring in some of the water from the large test tube.
- With one in each hand, hold the two test tubes angled toward each other with their tops touching. In one smooth motion, tip the small test tube up and into the large tube. The small tube should now be upside down inside the large tube. Expect to see an air bubble trapped in the small tube.
- Remove the air bubble by capping the large tube with a rubber stopper or cork. (Do not try for a very tight fit; forcing the stopper in too far may break the test tube.) Then slowly tilt the large tube so it is slightly past horizontal, which usually allows the air bubble to escape from the small tube. Slowly return the large tube to vertical and check to make sure the air bubble is completely gone from the small tube. Repeat this step if necessary.
With the Students - Day 1
- Give a simple explanation of how the class will try to indirectly observe yeast respiration by providing molasses as a source of sugar, and devising a way to try to capture the carbon dioxide gas that is given off as the yeast cells respire. Emphasize that the molasses is the food source that the yeast cells will convert to energy.
- Demonstrate how to set up the yeast respiration chambers, first using water to show how to remove air bubbles from the small test tube. Be sure to ask students why no air should be present at the onset. Next, demonstrate the same thing, using the 10% molasses solution instead of water. Caution students about pressing too hard on the stoppers.
- Show students the warm yeast solution, and demonstrate how to add yeast cells to the test chamber. This is done by first gently stirring the solution, then filling the pipette with some of it. (Since the yeast cells settle to the bottom of the beaker, it is important to stir the solution first.) Gently squeeze exactly 6 drops of the yeast solution down the side of the large test tube. Do not put the tip of the pipette into the molasses solution. Advise students to practice dropping 6 drops back into the beaker of yeast solution before attempting to add them to their test tubes. The drops should be uniform in size and not contain air bubbles.
- Demonstrate how to place a square of plastic wrap over the top of the chamber and smooth it around the tube's circumference. Point out that this keeps out insects and other airborne contaminants. Also demonstrate how to mark their chambers with initials or some other identification system. (This can be done at the onset if you prefer.)
- Give students some time to practice with water before they set up their test chambers. As students finish preparing their test tubes, have them carefully wipe any liquid from the outside of the tubes with damp paper towels, and place plastic wrap over the tops. Then, place the completed chambers in a test tube rack.
- When all students have finished, place the racks where they will not be disturbed. Avoid very warm or very cool locations.
With the Students - Days 2, 3 and 4 (and 5, if necessary)
With the Students - Concluding Discussion
- As students set up their respiration chambers, remind them not to push too hard on the rubber stoppers. If the test tube breaks, it could cut their fingers or hands. Be sure to have rubber gloves and clean paper towels on hand in case of accidents. Follow the First Aid procedures in place at your school.
- Do not allow students to taste the yeast-molasses solution at experiment end since it could be contaminated with other microbes.
- A student forgot to add yeast to his or her chamber. The yeast population should be visible as an opaque, tan-colored mass settled at the bottom of the large test tube.
- The yeast solution got too hot during the set-up procedures. If this happens, repeat the activity, carefully monitoring the temperature of the yeast solution during the preparation and setup procedures.
- The yeasts were stored or transported at extreme temperatures and killed prior to purchase. This is very unlikely. Nevertheless, you can test the yeast in advance. Simply follow the instructions for making the yeast solution; after 5-10 minutes you should see froth appearing on the surface of the solution. If not, try again with some new yeast. Be sure to discard any test solutions and start with fresh yeast solution on the day of the activity.
- Are all of the small gas bubbles produced by the yeasts trapped in the inverted test tube?
- Is the size of the bubble inside the inverted test tube an accurate representation of the amount of yeast respiration that occurred?
- Did the trapped gas bubble grow at a uniform rate each day? If not, why not?
- How do we know that the bubbles produced in the test tube were made of carbon dioxide?
- Did this experiment have a control? If not, what could be used as a control?
Mary R. Hebrank, project and lesson/activity consultant
© 2013 by Regents of the University of Colorado; original © 2004 Duke University
Engineering K-PhD Program, Pratt School of Engineering, Duke University
Last modified: November 26, 2015