Once these rates are determined the stoichiometric

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Unformatted text preview: as in a confined space with a device known as a CO2 gas sensor. This determination involves the measurement of the volume occupied by the gas of interest and then relating this volume to the number of moles of that gas. Another important feature of these equations is that they are in stoichiometric balance. This allows us to calculate the amount of any reactant or product if the amount of any other reactant or product is known. In our experiments, we will measure the rate of CO2 production in yeast cells undergoing alcoholic fermentation and the rate of CO2 production by corn seedlings undergoing cellular respiration. Once these rates are determined, the stoichiometric relationships of the reactions will be used to calculate the rates of ATP production for the organisms. The rate of ATP production is important because it closely approximates an organism’s rate of energy utilization. This physiological parameter is termed metabolic rate. The reason that the rate of ATP production is so closely related to metabolic rate is because ATP is not stored (to any significant degree) by cells. Therefore, for it to be produced it must be recycled through the energy-releasing conversion to ADP + Pi. In other words, if ATP is being produced, it is also being broken down. ***** Note: The questions addressed in this lab are listed on p. 5 of this exercise. As was true for Lab 5, information relevant to this week’s questions comes from several different areas of the text. In addition to the obvious search words/terms, the following will be helpful: aerobic/anaerobic metabolism, angiosperm life cycle, and seed germination. Biology 05LA – Fall Qtr. 2012 Lab 6 – page 2 The experiments in today's exercise will be done in groups of four, each member of which should have the opportunity to set-up one experiment. Each student will be responsible for all data collected by the group. FERMENTATION Yeast cells can carry out both cellular respiration and fermentation although the fermentative pathway is preferred by most yeast under anaerobic conditions. The yeast being used in today’s experiment, Saccharomyces cerevisiae, is unusual in that it will preferentially follow the fermentative pathway even when there is abundant oxygen to support the needs of cellular respiration. Because of this phenomenon (called the Crabtree effect) the amount of CO2 measured in this experiment will reliably reflect the CO2 being produced by fermentation. To measure the fermentation of glucose by yeast and the release of CO2 you will use a Vernier CO2 gas sensor inserted into an environmental chamber in which the yeast is fermenting. Since CO2 absorbs infrared radiation (IR) the gas sensor uses an LED to emit IR. A second internal sensor then determines how much of the IR is not absorbed by the CO2 in the environmental chamber. As the concentration of CO2 in the chamber increases, the amount of IR reaching the second sensor decreases. The CO2 gas sensor records the concentration of CO2 produced by the fermenting yeast in parts per million (ppm). Experimental protocol. A. Preparation of fermentation samples. 1. Set up the three fermentation samples in test tubes (17x150 mm) according to the values specified in the following table: Tube Label 1 2 3 0.3 / 20% 0.6 / 20% 0.6 / 5% Vol. and Conc. of Sugar 6 ml of 20% glucose 6 ml of 20% glucose 6 ml of 5% glucose Amt. Yeast 0.3 g 0.6 g 0.6 g 2. Seal each of the above tubes with Parafilm and set them aside to “activate”. (During this time the yeast cells hydrate and the cell’s metabolic machinery becomes functional). During this period you should swirl the cells as demonstrated by your TA to speed the activation process. It is critical that the cells not be allowed to clump at the bottom of the tube during activation; what we are striving for here is to create a suspension of individual yeast cells. Also, be careful to avoid placing the tubes in an excessively cool spot such as the draft from an air conditioning duct. B. Measurement of CO2 evolution. The following steps will be carried out separately for each of the 3 samples/tubes. You are now ready to begin measuring gas production by your samples. At this point you need to determine whether or not your samples are ready to measure. Measurement can proceed when the sugar/yeast mixture begins to bubble and the Parafilm bulges upward. Biology 05LA – Fall Qtr. 2012 Lab 6 – page 3 1. Turn on Lab Quest recorder by pressing the silver button on the upper left-hand side of the unit. 2. Pour the entire yeast mixture from the test tube into a petri dish and place the dish inside the large environmental chamber. Attach the cover which should have one opening already plugged with a rubber stopper. Insert the CO2 sensor into the second opening. 3. On the touchscreen of the LabQuest recorder, tap the icon in the upper left corner that looks like a speedometer with the stylus. Next tap “sensors” and then “data collection”. Change the “rate” to 0.1 sam...
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