An Aerobic Exercise: Yeast Metabolism with and without Aeration

Abstract

Objective

The objective of this experiment is to investigate yeast metabolism under aerobic and anaerobic conditions by measuring carbon dioxide output.

Credits

Andrew Olson, Ph.D., Science Buddies

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Introduction

Yeasts are single-celled fungi. Like the cells in your body, they can derive energy from sugar molecules. They can also break down larger carbohydrate molecules (like starches present in flour) into simple sugar molecules, which are then processed further.

Yeast can extract more energy from sugar when oxygen is present in their environment. In the absence of oxygen, yeast switch to a process called fermentation. With fermentation, yeast can still get energy from sugar, but less energy is derived from each sugar molecule.

In addition to deriving less energy with fermentation, the end products of sugar metabolism are also different. When oxygen is present, the sugar molecules are broken down into carbon dioxide and water (plus the energy that the yeast uses to grow and reproduce). In the absence of oxygen, the fermentation process produces alcohol, carbon dioxide and water (and less energy).

In this experiment, you'll grow yeast in containers with and without aeration, and compare the amount ofcarbon dioxide in the two conditions.

Terms and Concepts

To do this project, you should do research that enables you to understand the following terms and concepts:

• anaerobic,
• aerobic,
• glycolysis,
• fermentation,
• cellular respiration.

Bibliography

• Decelles, P. (2002, April 25). Cellular Respiration Overview. Retrieved October 22, 2009, from http://staff.jccc.net/pdecell/cellresp/respintro.html
• Thomson Gale, part of the Thomson Corporation. (2006). Respiration. Science of Everyday Things: Real-Life Chemistry, Volume 5. Retrieved October 22, 2009, from http://www.scienceclarified.com/everyday/Real-Life-Chemistry-Vol-5/Respiration.html
• Carter, J.S., 1996. "Cellular Respiration," University of Cincinnati, Clermont College [accessed January 13, 2006] http://biology.clc.uc.edu/courses/bio104/cellresp.htm

Materials and Equipment

For this project you will need the following items:

• dry yeast (buying a whole jar is probably more economical than individual packets),
• sugar (sucrose),
• warm water (typically 110°F–115°F, but consult the recommendations on your yeast package),
• thermometer to measure water temperature,
• 2 empty plastic bottles (1-liter soda bottles are good),
• 1 cap (must fit both bottles)
• one 3 foot piece of plastic aquarium tubing,
• aquarium aerator pump,
• disposable air stone,
• epoxy or silicone sealant,
• graduated cylinder (an empty plastic bottle can be substituted, if necessary),
• plastic tub or bucket,
• packing tape,
• water.

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Experimental Procedure

1. Do your background research.
2. You will be collecting CO₂ from the yeast by displacing water trapped in an inverted graduated cylinder. Here's how to set it up:
   1. Fill your plastic tub (or bucket) about one-third full with water.
   2. Fill the graduated cylinder with water.
      1. If your tub is big enough, fill the graduated cylinder by tipping it on its side inside the tub. Allow any bubbles to escape by tilting the cylinder up slightly, while keeping it under water. Keeping the opening of the cylinder under water, turn it upside down and attach it to the side of the tub with packing tape.
      2. If your tub is not big enough, fill the graduated cylinder completely and cover the top tightly with plastic wrap. Quickly invert the cylinder and place the opening in the tub, beneath the surface of the water. Remove the plastic wrap. Attach the cylinder to the side of the tub with packing tape.
   3. The graduated cylinder should now be upside down, full of water and with its opening under the surface of the water in the tub. It is ready to trap CO₂ produced by your yeast.
3. Next, you need a way to bring the CO₂ from the yeast to your gas collection apparatus.
   1. Make a hole in your bottle cap, just big enough to insert the plastic tubing. Get help from an adult if needed.
   2. Insert a piece of plastic tubing through the hole in the cap so that it sticks out about 2 cm. This will be the tube for collecting CO₂. It should remain above the surface of the yeast solution.
   3. Seal the tube to the cap with epoxy or silicone sealant so that it is air-tight. Allow the epoxy or silicone to cure fully before conducting your experiment.
4. When your gas collection apparatus is ready, you can start the actual experiment.
5. Label one bottle "+air" and the other bottle "−air".
6. You'll be making one solution at a time (unless you decide to set up more than one gas collection apparatus). It is important to use the same water temperature each time you make a solution, since yeast activity is temperature-dependent.
7. Boil 3 cups of water (to minimize dissolved oxygen), and let the water cool to between 43–46°C (about 110–115°F). We'll call this "warm, deoxygenated water" in the rest of the procedure.
8. Dissolve 4 tablespoons of sugar in 2 cups of warm, deoxygenated water. Stir slowly and gently. When the sugar is fully dissolved, add 2 teaspoons of yeast (this is about the same amount as 1 packet of yeast), mix and pour into "−air" bottle. Leave space at the top of the bottle, so that your CO₂ collection tube remains above the yeast solution. Be sure to note the actual temperature of the water in your lab notebook.
9. Cap the bottle tightly with your "tube cap," and place the open end of the collection tube inside your gas collecting cylinder. Note the starting time in your lab notebook.
10. For the "+air" condition, before adding the yeast to the sugar water, aerate the solution with the aquarium pump and airstone.
11. After 5 minutes, stop aerating and add the yeast. Pour the solution into the "+air" bottle and cap it, placing the open end of the collection tube inside your gas collecting cylinder. Note the starting time in your lab notebook.
12. Within 5–10 minutes, the yeast solution should start foaming, and you should see bubbles collecting in the graduated cylinder. Note the time when you first start seeing bubbles in your lab notebook.
13. To promote oxygen circulation in the yeast solution, for the "+air" bottle only, gently "swirl" the bottle to stir the contents.
14. Decide how long to collect CO₂ (somewhere between 30–60 minutes is probably good, but you may need to adjust for your particular conditions). Use the same amount of time for all of your tests.
15. When the time is up, note how much CO₂ was collected.
16. Re-fill your gas collection cylinder, and carefully rinse out the yeast solution from the bottle. You should run at least three separate trials for each condition.
17. Calculate the average volume of CO₂ produced under each condition.

• Which condition produced more CO₂? What is the ratio of CO₂ production between the two conditions? Is this consistent with your expectations from your background research?

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Variations

• For a more advanced project, investigate what happens if you double the amount of sugar. You'll need to do more background research on yeast metabolism for this project.
• For another project that explores CO2 production in yeast, take a look at œYeast Reproduction in Sugar Substitutes.

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