Yeast are able to metabolize some foods, but not others. In order for an organism to make use of a potential source of food, it must be capable of transporting the food into its cells. It must also have the proper enzymes capable of breaking the food’s chemical bonds in a useful way. Sugars are vital to all living organisms. Yeast are capable of using some, but not all sugars as a food source. Yeast can metabolize sugar in two ways, anaerobically, without the aid of oxygen, or in the case of our experiment aerobically, with oxygen. Without oxygen present, fermentation occurs. Fermentation is a process by which a living cell, such as yeast, obtains energy through the breakdown of glucose and other simple sugars.

In this experiment, we will try to determine whether yeast are capable of metabolizing a variety of sugars. When yeast respire aerobically, oxygen gas is consumed and carbon dioxide, CO2, is produced. We will use the height of the foam as an indicator in order to monitor the production of carbon dioxide as yeast respire using different sugars. The bubbles in the foam are full of the carbon dioxide created by the yeast’s respiration. By comparing the heights of the foam, we can conclude the relative rates of respiration. The four sugars that will be tested are glucose (blood sugar), sucrose (table sugar), fructose (fruit sugar), and lactose (milk sugar).

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1. Prepare a water bath for the yeast. A water bath is a device that maintains water at a constant temperature. It is used in the microbiological laboratory for incubations. At the beginning of the lab period, you should check the water bath to see if it is turned on, set at the right temperature, and filled with water. Water baths should be filled with distilled water. If you are using the water bath for an experiment you should check the temperature frequently to make sure that the water bath is maintaining the proper temperature. This ensures that the yeast will remain at a constant and controlled temperature. To prepare the water bath, obtain some warm and cool water from your teacher. Combine the warm and cool water in the 600 mL beaker until it reaches 38–40°C. The beaker should be filled with about 300–400 mL water. Leave the thermometer in the water bath during the course of the experiment to monitor the temperature of the water bath.
2. Obtain five test tubes and label them G, S, F, L, and W.
3. Obtain the four sugar solutions: glucose, sucrose, fructose, and lactose. a. Place 2 mL of the glucose solution in test tube G. b. Place 2 mL of the sucrose solution in test tube S. c. Place 2 mL of the fructose solution in test tube F. d. Place 2 mL of the lactose solution in test tube L. e. Place 2 mL of distilled water in test tube W.
4. Obtain the yeast suspension. Gently swirl the yeast suspension to mix the yeast that settles to the bottom. Put 2 mL of yeast into each of the five test tubes. Gently swirl each test tube to mix the yeast into the solution.
5. Set the five test tubes into the water bath.
6. Incubate the test tubes for 10 minutes in the water bath. Keep the temperature of the water bath constant. If you need to add more hot or cold water, first remove as much water as you will add, or the beaker may overflow.
7. When incubation is finished get a ruler and proceed to measure the height of the foam produced within each of the test tubes and record your data down into a book.
8. Repeat Steps 1-7 for the other four test 3 times to obtain an overall average and exclude any anomalies you may have found to make your results more accurate.


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