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TEACHER REFERENCE PAGES - YEAST FERMENTATION LAB Biology Yeast Lab, Page 7 4/26/01 Introduction Most organisms, including yeasts, use oxygen in a process called cellular respiration. Cellular respiration is the controlled breakdown of carbohydrate to carbon dioxide and water with capture of some of the energy in the form of ATP. The rest of the energy is lost in the form of heat. The first stage of the breakdown is called glycolysis and the second stage is called the Krebs Cycle. During this process, electrons are transferred from the carbohydrates to oxygen in the process called electron transport and water is formed as the final product of electron transport. Electron transport produces a chemosmotic gradient of protons (H+) and positive charges across a membrane and this gradient can drive the formation of ATP. Cellular respiration produces approximately 38 ATP molecules from each molecule of the sugar glucose that is broken down. The carbon that was in the carbohydrate is fully oxidized to form CO 2 during respiration. For glucose, the 6 carbons become 6 CO 2 molecules. Table 1. Comparison of respiration and fermentation of glucose in yeast. PROCESS CONDITION S PRODUCTS FROM GLUCOSE AMOUNT OF ATP RESPIRATION AEROBIC 6 CO 2 +6H 2 38 FERMENTATION ANAEROBIC 2 CO 2 +2C 2 H 6 O2 Fermentation , a process that can occur in the absence of oxygen, partially breaks down carbohydrate by glycolysis to capture a small amount of energy in the form of ATP. The initial reactions of fermentation and respiration are the same, but fermentation stops after glycolysis whereas respiration continues into the Krebs Cycle. The carbohydrate leftovers are different depending upon the organism that performs the fermentation; usually one product is more oxidized (electron-poor) than the starting molecule and the other is more reduced (electron-rich). In the case of yeast fermentation, the products from one glucose (C 6 H 12 O 6 ) molecule are two molecules of ethanol (C 2 H 5 OH) and two molecules of CO 2 . Human anaerobic (oxygen-free) muscle produces two molecules of lactic acid (C 3 H 6 O 3 ). Even though the products are different, each fermentation results in a limited, anaerobic breakdown of carbohydrate with energy release. Since the process does not completely break down the carbohydrate, it does not release much energy that can be captured in the form of ATP. In yeast
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TEACHER REFERENCE PAGES - YEAST FERMENTATION LAB Biology Yeast Lab, Page 8 4/26/01 fermentation, there are 2 ATP molecules produced for each glucose molecule that is fermented. This is a low yield compared to that of respiration, but the ability to perform fermentation allows the yeast to survive and grow in environments where no oxygen exists (see Table 1). Gas Chromatography The major technique that is used to determine the type of organic molecules produced during fermentation is gas chromatography. Gas chromatography (GC) is the separation of compounds in the gas phase, depending on their relative ability to
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