Yezi Cho
American Community School of Abu Dhabi
1. Fermentation
1.1. Cellular Respiration
Cellular respiration is the process of breaking down glucose to produce adenosine triphosphate (ATP), an energy-carrying molecule. The main steps of cellular respiration are glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation coupled with chemiosmosis. The last step - oxidative phosphorylation - is a process including the electron transport chain. After the electrons are transferred from one molecule to another, releasing energy, oxygen is the molecule that accepts the electrons and allows the continuation of ATP production. Therefore, under circumstances where there is no oxygen, this process is not possible. In this case, yeasts undergo fermentation - an anaerobic process of breaking down glucose for energy. During fermentation, the yeast cells produce ATP during glycolysis, a process in which oxygen is not a contributing factor, and repeat the process through NAD+ regeneration. Classified according to the resulting product, alcohol (ethanol) fermentation, lactic acid fermentation, and butyric acid fermentation are several examples of fermentation. In this study, alcohol fermentation takes place, producing ethanol and carbon dioxide as the byproduct.
1.2. Glycolysis
Among the steps of cellular respiration, glycolysis is the initial stage that produces two net molecules of ATP. In simple terms, two phosphate molecules derived from two ATP molecules are used to modify a glucose molecule into the fructose-1,6-bisphosphate molecule; this unstable molecule is then split to form two three-carbon sugars. Each three-carbon sugar is converted to two pyruvates, each producing two ATP molecules and one NADH molecule. Therefore, two ATP molecules and two NADH molecules are produced.
In alcoholic fermentation, the electrons of NADH are used to produce ethanol from pyruvates. In detail, a carboxyl group is disconnected from the pyruvate and becomes carbon dioxide, while the pyruvates become two acetaldehyde molecules. The electrons from two NADH molecules turn acetaldehyde into ethanol, and the NAD+ produced can be used for further fermentation.
2. Materials and Methods
2.1. Sugar
Sugar is one of the reactants of cellular respiration that are broken down for energy that powers the body. It is a type of carbohydrate - a molecule formed from carbon, hydrogen, and oxygen. Carbohydrates can be classified into monosaccharides, disaccharides, and polysaccharides - one simple sugar, two simple sugars, and a polymer of sugars respectively. The monosaccharides are the basic compounds of sugar. Glucose, fructose, and galactose are isomers - compounds that are formed from the same number and type of atoms but differ in the arrangement of atoms. Due to the identical molecular formula, C₆H₁₂O₆, the monosaccharides share a common molar mass of approximately 180.16g/mol. Disaccharides are formed by two monosaccharide units combined by a dehydration synthesis reaction. During the process, a water(H₂O) molecule is released, in turn resulting in a glycosidic bond in the disaccharide. Sucrose, lactose, and maltose are respectively formed from glucose and fructose, glucose and galactose, and two glucose units. Their molar mass is 342.3g/mol - twice the molar mass of two monosaccharides reduced by the molar mass of water(18g/mol).
S. Cerevisae’s cellular respiration process was tested using different types of sugars. In total, four types of sugars were used: two monosaccharides - glucose and fructose, and two disaccharides - sucrose and lactose. Though cellular respiration is widely known as a process of breaking down glucose, this study also experiments on sugars that are usually not considered direct reactants. Therefore, the following hypothesis is possible: among the certain types of sugar that enable fermentation, the rate of fermentation will vary due to the different enzyme catalysis and metabolic pathways yeast cells undergo.
2.2. Measures and Procedures
The rate of fermentation was measured using a Vernier gas pressure sensor and its graph was analyzed using LoggerPro. For each trial, a solution consisting of sugar and yeast was added into a test tube. 3 mL of yeast solution and 3 mL of 5% sugar solution were swirled to mix, and the solution was incubated in a water bath for three minutes. Using a thermometer and hot water, the water bath was maintained between 35-38°C. After incubation, the gas pressure sensor was added and data was collected for another three minutes.
3. Results
While the graphs below show a record of the minimum and maximum gas pressures, the minimum gas pressure does not accurately represent the starting gas pressure of the analysis. The gas pressure sensor was started before the rubber cork was added onto the test tube, thus the initial gas pressure was determined according to the data recorded every 0.5 seconds (not shown in the photos below).
Figure 1. Glucose, trial 1
Figure 2. Sucrose, trial 1
Figure 3. Fructose, trial 1
Figure 4. Lactose, trial 1
Figure 5. Glucose, trial 2
Figure 6. Sucrose, trial 2
Figure 7. Fructose, trial 2
Figure 8. Lactose, trial 2
Figure 9. Gas pressure inside test tube (kPa)
Name of Sugar | Gas Pressure inside test tube (kPa) | kPa | |
Trial 1 | Trial 2 | Mean | |
Glucose | 1.3 | 0.8 | 1.1 |
Sucrose | 2.5 | 1.2 | 1.85 |
Fructose | 1.6 | 1.7 | 1.7 |
Lactose | 0.5 | 0.2 | 0.4 |
Initially, the yeast conducted aerobic respiration, using sugar and the oxygen inside the container to produce water and carbon dioxide. After all of the oxygen was depleted for respiration, anaerobic respiration took place, producing carbon dioxide and ethanol as the byproduct. After one hour of observation, whether the yeast fermented using each type of sugar could readily be determined by the volume of carbon dioxide produced. All of the sugars resulted in production of carbon dioxide, but the rate differed according to each sugar. This can be interpreted as yeast having the specific enzymes that catalyze the fermenting process for each of the sugars.
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