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AP Biology: Fermentation, Respiration, & Enzyme Specificity Introduction: All living cells, including the cells in your body and the cells in yeast, need energy for cellular processes such as pumping molecules into or out of the cell or synthesizing needed molecules. ATP is a special molecule which provides energy in a form that cells can use for cellular processes. Cellular respiration is the process that cells use to transfer energy from the organic molecules in food to ATP. The following equation summarizes the chemical changes that occur in cellular respiration of the monosaccharide glucose when oxygen is available:

C6H12O6 + 6 O2 ! 6 CO2 + 6 H2O + (up to) 38 ATP

The chemical reactions in cellular respiration are similar to the chemical reactions when organic chemicals are burned, but of course no ATP is produced. Instead energy is released in the form of light and heat. The following equation shows the chemical changes that occur when the monosaccharide glucose is burned. Consider the similarities and differences between these equations:

C6H12O6 + 6 O2 ! 6 CO2 + 6 H2O + light + heat

There is another important feature of cellular respiration which is not shown in these equations. Cellular respiration involves many small steps; these multiple steps allow the cell to use the energy from each glucose molecules efficiently in order to make as many ATP molecules as possible. The multiple steps of cellular respiration are described in your book and notes. This lab focuses on how yeast perform cellular respiration in the absence of oxygen. Remember if oxygen is present, glycolysis is followed by the Krebs cycle and electron transport chain, or aerobic respiration. When oxygen is not present, glycolysis is followed by anaerobic respiration, aka fermentation. Glycolysis is an anaerobic process. It occurs regardless of the presence or absence of oxygen. There are two types of fermentation, alcoholic and lactic acid. The names come from the fate of the pyruvic acid formed at the end of glycolysis. Which type of fermentation occurs depends on the organism. The disadvantage of fermentation is that it produces fewer ATP molecules than aerobic respiration. The advantage, however, is that glycolysis can continue, so at least some ATP is still being produced. The study of fermentation and respiration by yeast is a powerful tool for presenting important aspects of metabolism. In this experiment, you will be using a simple fermentation chamber to garner meaningful insights into metabolic processes using yeast as the organism of study.

A number of instruments have been used to quantify CO2 production during microbial alcoholic fermentation. These devices include U-shaped fermentation tubes, inverted pipets and water displacement apparatuses. These systems invariably present problems in volume measurements due to foaming, leaking, or long lag times, and expensive equipment. You will use an inexpensive and versatile fermentation tube that allows experiments to be conducted in as little as 15 minutes. Purpose: To provide the backdrop for productive discussions of the nature of catabolism, bioenergetics, and metabolic controls. Materials: 4, 15 mL centrifuge tubes 7% yeast suspension distilled water 1%, 5%, 10% sucrose solution Procedure: The fermentation tube is a plastic, 15 mL centrifuge tube with volume gradations and screw on cap that has been perforated with holes.

1. Fill a tube half way (8 mL) with 1% sugar solution and the remainder of the tube with a 7% yeast suspension. When filling, extend the fluid meniscus above the lip of the tube.

2. Screw on the cap and conform that any sizable air bubbles have been excluded. 3. Invert the tube and place in a large beaker filled with 40o C water. Place this

beaker in a water bath to maintain a constant temperature. 4. At appropriate time intervals, mark the bottom of the CO2 bubble (include any

foam as part of the bubble) with a permanent marker. Periodically, the tube should be mixed by gentle inversion.

5. At the end of the experiment, empty the tube and record the mL of CO2 that correspond to the pen markings.

6. Repeat this procedure for 5% sugar solution, 10% sugar solution, and distilled water.

Hypothesis Write your hypothesis in the space below. Remember that a hypothesis is a testable model/explanation based on your prior knowledge about the experiment topic. Your prediction below will be your guess about how the experiment will turn out. ______________________________________________________________________ ______________________________________________________________________

Prediction Write your prediction of the outcome of the experiment in the space below. ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________

Data:

Volume of CO2 Bubbles: Sucrose Treatment

0%

1%

5%

10%

Graph your Results! Conclusion Questions:

1. What is sucrose?

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2. What was the independent variable in your experiment? (1 pt)

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3. What was the dependent variable in your experiment? (1 pt)

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4. What was the control in your experiment? (1 pt)

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5. What was the purpose of the control? (2 pts)

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6. Did the yeast produce different amounts of carbon dioxide with different sucrose

concentrations? Why do you think this happened? (3 pts)

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7. Was your prediction correct? Did the data support your hypothesis? Explain.

Would you revise your hypothesis? If so, why? (4 pts)

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8. If you were to continue your investigation of yeast, what would you test next?

Give a brief description of the experiment you would do. (3 pts)

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9. What is the “Pasteur Effect”? ______________________________________________________________________