Yeast Fermentation and Respiration

Yeast Fermentation and Respiration Exercise #9 Introduction Yeast is a fungus capable of converting sugars into alcohol and carbon dioxide (CO2) by the process called alcoholic fermentation. Alcoholic fermentation is a common reaction carried out in a wide range of settings from homes for baking bread to factories for brewing beer. It is defined as the conversion of glucose into energy, ethanol, and CO2. It works by starting with glycolysis, which is the first metabolic pathway involved in cellular respiration. Glycolysis initiates glucose to break down into 2 pyruvate ions. This yields 2 ATP and 2 NADH molecules which both play a key role for processes that require energy. Then, fermentation finally occurs in the absence of oxygen, or anaerobic environments. The pyruvate ions convert into 2 CO2 and 2 acetaldehyde molecules. The 2 NADH molecules oxidize into NAD+ and acetaldehyde reduces into ethanol by accepting electrons from NADH. The purpose of this experiment is to observe the formation of CO2 in varying concentrations of yeast and glucose and adding heat in an anaerobic environment. The test tube with the highest yeast and glucose concentration (i.e. Tube 4) will have the tallest height, indication the most CO2 formation. Methods Four test tubes labeled 1-4 were filled with varying amounts of deionized water, yeast, and glucose, as detailed in Table 1, then covered with parafilm. The tubes were fitted into flasks that had been placed into a hot water bath at 30 °C. The amount of CO2 evolved was recorded every 2 minutes for 20 minutes total. This was done by measuring the height of the solution (cm) from its initial starting point to its terminal point at the end of each interval. Table 1. Solutions within tubes 1-4. tube DI H2O (mL) yeast (mL) glucose (mL) 1 6 0 6 2 6 6 0 3 3 3 6 4 1 5 6 Results In order to determine the CO2 production rate, the amount of CO2 evolved in each tube was measured from its starting point in 2 minute intervals for 20 minutes. The results are shown in Graph 1. The volumes ranged from 0.00-0.55 cm with Tube 1 containing the lowest amount of CO2 formation and Tube 4 containing the highest amount of CO2 formation. Graph 1. Amount carbon dioxide evolved (cm) in test tubes Discussion Tube 4 produced the highest amount of CO2 because it contained the most yeast in combination with glucose. This means that Tube 4 contained the highest fermentation rate which allows the hypothesis to be accepted. An important result to discuss is the observation of CO2 formation in Tube 2. According to Table 1, there was no glucose present. Therefore, the formation of CO2 should not have been possible because there is no compound for yeast to react with. A possible explanation could be that upon pouring yeast into all of the tubes, the tip of the graduated cylinder being used was contaminated with glucose from other tubes. Although this result was not ideal, this minor contamination does not significantly skew other results. By observing and measuring CO2 formation, the experiment illustrated the process of yeast fermentation. The lack of yeast in Tube 1 and the lack of a significant amount of glucose in Tube 2 represented the control groups as well as the need for both reactants to be present in a solution in order for fermentation to occur and produce CO2. The observation that Tube 4 yielded higher CO2 volume than Tube 3 demonstrates that the rate of fermentation increases when the amount of yeast in oxygen-deprived solutions increases. This is because the amount of limiting reactant in the solution is increased therefore generating a higher product yield. Tinkering with yeast and sugar concentrations allows for a multitude of varying alcohols and breads to contribute to the world’s current stock of recipes. Yeast is an important compound that has been adapted by many in order to produce goods that are unique to countries, cities, and even families for bakers at home.