My Science Box

Summary
Bubbling Yeast: Thanks to Ellen Loehman for creating this image.Yeast are a single celled fungi that are a great model organism for studying respiration in the classroom. The species Saccharomyces cerevisiae is commonly used for leavening bread and fermenting beer but other species such as Candida albicans are known to cause infections in humans (vaginal yeast infections and diaper rash being the most common). In this investigation, students fill the bulb of a disposable pipet (eyedropper) with yeast, then submerge the pipet in a test tube of water. They can then measure the rate of respiration by counting the number of bubbles of carbon dioxide gas that emerge from the tip of the pipet in a certain length of time. By varying the temperature and the nutrient source, students can discover what variables affect the rate of respiration in yeast. By submerging the pipet in bromthymol blue (see Colorful Respiration activity), students can identify the gas being produced as carbon dioxide.

For more information about yeast in classroom experiments, see this experiment from the Exploratorium or this one from PBS Kids that has snippets of what different kids saw with different manipulations. The idea of using inverted disposable pipets to contain the yeast and measure their respiration rate came from a workshop led by Steve Ribisi of the University of Massachusetts – thanks Steve!

Materials

• Fast-acting bread yeast (1 packet or 1/4 teaspoon per group)
• 1 cup water
• 2 tablespoons table sugar
• Disposable plastic pipets (These can be ordered from most science supply companies such as Science Kit and Boreal Labs, around $6 for 100 pipets or $18 for 500 pipets. Note: don’t get the “microtip” style since the yeast solution is too viscous to be sucked into the tip.)
• Metal washers from the hardware store (These will weigh the pipets down so that they don’t float up to the top of the tube. Make sure that the hole in the washers is large enough to sit around the neck of the pipet and rest on top of the bulb.)
• Small test tubes for mixing yeast solution with sugar solution
• Large test tubes (These must be wide enough to accommodate the pipets and washers comfortably but also tall enough to submerge the whole pipet. If you don’t have large enough test tubes, try using graduated 15 ml centrifuge tubes, a 100 ml graduated cylinder, or a small beaker.)
• Optional: other nutrient sources for the yeast such as milk, apple juice, soda, Kool-aid, salt water, potato starch solution, flour in water, chicken broth, etc. Most of these work better if diluted in water 1:1.
• Optional: bromthymol blue solution

Procedure

1. an hour before the activity, mix 1 packet of bread yeast with 1/4 cup of luke warm water. Stir around 2 minutes until all the yeast is dissolved. Stir again just before use.
2. Dissolve 1 tablespoon sugar in 1/2 cup of luke warm water. Stir around 1 minute until all the sugar is dissolved.
3. In a small test tube, mix equal quantities of the yeast solution and sugar solution. Stir gently to combine. Use separate droppers for each solution to avoid contaminating the original stock solutions.
4. Suck up some of this solution into a pipet. Invert the pipet and let the solution run down into the bulb. Carefully squeeze out the air and suck up some more yeast-sugar solution. Try to fill exactly half of the pipet bulb.
5. Thread 2 washers over the neck of the pipet so that they come to rest on top of the bulb.
6. Gently drop the pipet with washers into the large test tube.
7. Fill the large test tube with luke warm water until the pipet is completely submerged.
8. Wait 5 minutes to allow the yeast time to equilibrate and begin respiration.
9. Count how many bubbles emerge from the top of the pipet each minute for 10 minutes.

Going Further

1. Investigate the effect of temperature. Compare the respiration rate in yeast in cold water, luke warm water, and scalding hot water.
2. Investigate the effect of different nutrient sources. Compare the respiration rate in yeast dissolved in different nutrient sources.
3. Investigate the identity of the gas produced. Fill the test tube with BTB rather than water and see what happens to the color of the indicator over time. (See the Colorful Respiration activity.)

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