Background:

All life forms need to convert or transform energy-rich organic molecules like sugars into forms of energy that can be used to perform cell work. Cellular respiration is the set of chemical reactions during which molecules are broken down to release energy. For example, yeast cells break down sugar (sucrose) in grape juice. They produce carbon dioxide gas and alcohol (ethanol).

Cellular respiration is controlled by a series of enzymes. The enzymes that help this system of chemical events are often sensitive to physical and chemical conditions such as temperature, solute concentration, and pH.

In this activity, you will use a pressure sensor to measure the change in pressure in a flask containing a mixture of activated yeast and grape juice. Then you will repeat the measurement for a mixture of activated yeast, grape juice, and a small amount of a chemical – sodium fluoride.

The Question:

What factors can alter the rate of the fermentation of grape juice?

Variables:

Identify the type of data you will collect to support your hypothesis and state the manipulated, responding and controlled variables in this investigation.

SAFETY REMINDER: Wear protective equipment while handling chemicals.

Materials:

• pressure sensor (absolute) (comes with plastic tubing and connectors)
• temperature sensor
• 2 USB Links or a PowerLink
• beaker (250 ml)
• graduated cylinder
• flask (250 ml) with one hole rubber stopper
• hot plate
• magnetic spinner with stir bar
• meter stick
• triple beam balance or electronic scale
• glycerin
• grape juice
• sodium fluoride
• yeast suspension (Dissolve 1 teaspoon table sugar (sucrose) in 200 mL warm water (approximately 40°C, or 100°F - 110°F). Add one envelope (approximately 7 grams) dry yeast (Fleishmann's Active Dry Yeast or similar product) and mix well. This 200-mL suspension will be enough for four lab groups. Do not prepare this ahead of time! Prepare the suspension just before students are ready to collect data so that activation is just beginning.

Procedure:

Step 1:

Step 2:

Upon opening the DataStudio file, you will be presented with a pressure-time graph that looks something like this:

The pressure sensor will record data at a rate of one sample every five seconds.

Step 3:

Put 150 mL of grape juice in a beaker. Place the beaker on a hot plate. Place the Temperature Sensor in the grape juice.

Turn on the hot plate. Warm the juice to a temperature of 35&Mac251; Celsius. Use the software and a digital display to monitor the temperature of the grape juice:

Do not warm the grape juice above 40°C. The yeast will begin to die at around 42°C. The optimum temperature for the yeast is around 35°C.

Step 4:

Put a drop of glycerin on the barb end of the quick-release connector and insert the barb into one end of the plastic tubing.

Put a drop of glycerin on the smaller diameter end of the connector that will go into the rubber stopper. Insert the small diameter end into the plastic tubing:

Put a drop of glycerin on the larger diameter end of the connector that will go into the rubber stopper, and insert the end into the rubber stopper.

Align the quick-release connector on the end of the plastic tubing with the connector on the pressure port of the pressure sensor. Push the connector onto the port, and then turn the connector clockwise until it clicks (about one-eighth turn).

Step 5:

Remove the Temperature Sensor from the beaker. In the software, stop monitoring the temperature of the grape juice.

Transfer the warmed grape juice to the flask. Add a spin bar to the flask.

Gently add 10 mL of yeast suspension to the juice.

Stopper the flask with the rubber stopper. Use a twisting motion to get a tight fit. Place the flask on the magnetic stirrer. Turn on the stirrer and adjust to a moderately fast stirring rate:

Start recording data by pressing the DataStudio start button:

Allow the yeast to metabolize the grape juice for about 40 minutes and then stop recording data.

Carefully remove the rubber stopper from the flask and dispose of the grape juice/yeast mixture as instructed by your teacher.

Step 6:

Repeat steps 1-5, but this time add 1.0 g of sodium fluoride to the beaker along with the grape juice in step 3.

Analyzing and Interpreting:

1.	Construct a data table that will allow you to note the relevant times and pressures from the graph:

Set up your Graph display so it shows your data.

Use the Graph display’s built-in statistics to determine the Ending Pressure, the Starting Pressure, the Ending Time and the Starting Time for the first run of data. Do this by clicking the 'Statistics Menu' button: and select all the above.

Record the minimum X as Starting Time in the Data Table in the Lab Report section. Record the maximum X as Ending Time.

Record the minimum Y as Starting Pressure in the Data Table. Record the maximum Y as Ending Pressure.

Calculate the difference/change in pressure and record it in the Data Table. Calculate the difference/change in time and record it in the Data Table.

Calculate the rate of production of carbon dioxide by the yeast from grape juice and record the rate.

Divide the difference in Pressure readings by the difference in Time:

2.	How does the pressure inside a closed vessel change as yeast converts the sucrose in grape juice into ethanol and carbon dioxide?
3.	What is the rate of production of carbon dioxide gas for the grape juice and yeast mixture?
4.	What happens to the rate of carbon dioxide production in the flask when the sodium fluoride is added to the grape juice?

Forming Conclusions:

5.	Based on the data you have collected, write a summary statement for the following questions: What factors can alter the rate of the fermentation of grape juice?

Extending:

Explore other factors that may affect this particular chemical reaction by changing other variables, for example:

• pH level (for example, you could 25 mL of any of the following buffer solutions to 125 mL of grape juice: (a) pH buffer 2, (b) pH buffer 3, (c) pH buffer 4, and (d) pH buffer 6. (e) pH buffer 10
• yeast concentration (for example, you could vary the amount of yeast by mixing and using suspensions of 100 mL of water and (a) 1 g, (b) 3 g, (c) 6 g, (d) 12 g, or (e) 20 g of dry yeast.