Respiration in Yeast

Evidence for: HS-LS2-3. Construct and revise an explanation based on evidence for the cycling of matter and flow of energy in aerobic and anaerobic conditions. HS-LS1-7. Use a model to illustrate that cellular respiration is a chemical process whereby the bonds of food molecules and oxygen molecules are broken and the bonds in new compounds are formed resulting in a net transfer of energy. Adapted from http://www.sciencebuddies.org/science-fair-projects/project_ideas/MicroBio_p009.shtml#summary

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 of carbon dioxide in the two conditions. You will be collecting CO2 from the yeast by displacing water trapped in an inverted graduated cylinder.

Materials  Graduated cylinder (at least 100 mL)  Empty plastic bottle, such as a bottled water  Plastic tubing, approximately three times as long as container (at least 500 mL) the height of the plastic tub or bucket used  A cap that fits the bottle  Thermometer  Drill or a nail and a hammer  Aquarium aerator pump  Epoxy or silicone sealant that works with plastics  Air stone  Permanent marker  Dry yeast.  Measuring spoon  Plastic tub or bucket  Measuring cup  Water  Warm water (recommended on the yeast package)  Plastic wrap  Sugar  Packing tape   Setup 1. Fill your plastic tub (or bucket) about one-third full with water. 2. Fill the graduated cylinder with water. a. If your tub is big enough, fill the graduated cylinder by tipping it on its side inside the tub. a.i. Allow any bubbles to escape by tilting the cylinder up slightly, while keeping it under water. a.ii. Keeping the opening of the cylinder under water, turn it upside down and attach it to the side of the tub with packing tape. b. If your tub is not big enough, fill the graduated cylinder completely and cover the top tightly with plastic wrap. b.i. Quickly invert the cylinder and place the opening in the tub, beneath the surface of the water. b.ii. Remove the plastic wrap. b.iii. 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 CO2produced by your yeast.

4. Next, you need a way to bring the CO2 from the yeast to your gas collection apparatus. a. Make a hole in your bottle cap, just big enough to insert the plastic tubing. Use a drill or a nail and a hammer. b. 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. c. 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. d. After the epoxy or silicone has completely cured, which should take approximately one day at most, check to make sure the tube and cap are sealed together with an airtight seal or you may have a leak that could affect your results. If there is a leak, apply more epoxy or silicone sealant and let it completely cure again.   Recording data 1. The conditions you will be testing are with oxygen and without oxygen. You will do at least three trials for each condition. Make a data table to record this raw data. Read through the procedure fully to determine any other data that must be collected. 2. Test the yeast with a solution that has oxygen. a. Boil 3 cups of water and let the water cool to between 43–46°C (about 110–115°F). b. Dissolve 2 tablespoons (Tbsp.) of sugar in 2 cups of warm water. Stir slowly and gently. c. When the sugar is fully dissolved, aerate the solution with the aquarium aerator pump and airstone. d. After 5 minutes, stop aerating the solution. e. Next add and mix in 2 teaspoons (tsp.) of yeast.

f. Pour the entire solution into the 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. g. 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.

h. There should be water in the tubing as soon as it is submerged in the water. The CO2 gas will push some water out

of the tubing before the graduated cylinder starts to fill with CO2 gas. i. Within 5–10 minutes, the yeast solution may start foaming, and you may see bubbles collecting in the graduated cylinder. If you observe them, note the time when you first start seeing bubbles. j. To promote oxygen circulation in the yeast solution, periodically gently "swirl" the bottle to stir the contents.

k. 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.

l. Note: Do not let the graduated cylinder become completely filled with CO2, but instead stop it before this point. If

you let it become completely filled, and the next condition you test makes even more CO2, this could lead to poor and inaccurate results because your graduated cylinder may fill up before your test time is over.

m. Tip: If your solution makes a large amount of CO2 very quickly, you can try to make it produce less CO2 by using less sugar and possibly less yeast. For example, you could repeat this step using 1 tsp. sugar (instead of 2 Tbsp.) and 1 or 2 tsp. yeast (instead of 2 tsp.).

n. When the time is up, note how much CO2 was collected. Record your results in the data table. 3. Repeat steps from the set-up to re-fill your gas collection cylinder and reset your gas collection apparatus. Carefully rinse out the yeast solution from the bottle. 4. Repeat steps “a-n” at least two more times. 5. Repeat steps “a-n” but this time test the solution without oxygen by making the following changes: a. Skip steps “c-d” so that you do not aerate the solution. b. Boiling the water should have minimized the dissolved oxygen in the water. c. Skip step “j”, as you do not want to promote oxygen circulation this time. d. Be sure to note the starting time and, if you observe them, note the time when you first start seeing bubbles.   Assessment  Create a poster with each of the following parts to demonstrate your understanding of the standards listed above. A. Calculate the volume of gas that is produced in each trial. Show a sample calculation of how you determined the volume and explain why this best represents the data. Add an appropriately labeled section to the data table to put on the poster.

B. Calculate the average volume of gas collected for each condition and determine the ratio of CO2 production. Add an appropriately labeled section to the data table to put on the poster. C. Graph this processed data. Title this graph and label the axes. D. Annotate a drawing of the lab explaining how this lab demonstrates the cycling of matter and flow of energy in aerobic and anaerobic conditions. E. Connect the evidence to your explanations. F. Compare the amounts of ATP generated in each type of respiration to your calculated ratio, evaluate the results. 