Effect of Type of Sugar on Glycolysis and Fermentation in Yeast
Introduction:
Many cells go through glycolysis and fermentation to break glucose down to carbon dioxide and ethanol (Black et al. 158 and 172). In glycolysis, glucose is converted to pyruvate, and in fermentation, pyruvate is converted to carbon dioxide and ethanol (Black et al. 158 and 172). Glycolysis and fermentation provide these cells, such as yeast, in this experiment, with energy (Peck). However, yeasts do not utilize all kinds of sugar in the same rate. Some sugars can even not be utilized by yeasts (Peck).
This experiment aimed to study whether certain types of sugars can be utilized by yeast, if so, what is the different rate that the yeast utilize them. The rate was studied by measuring the rate that the systems produced carbon dioxide, one of final products of fermentation. The sugars being investigated were fructose, galactose, sucrose, maltose, and sactose. Rate that yeast utilized different sugars were compared with the rate that the yeast utilized glucose. The group of yeasts treated with glucose was positive control and that of yeast not treated by any sugar was negative control.
In glycolysis, glucose is turned to fructose-6-phosphate, which is the phosphorylated form of glucose (Black et al. 159). Therefore, we expected that yeast would utilize fructose in similar rate as it utilizes glucose. Galactose is a geometric isomer glucose, which is not involved in the process of glycolysis. Hence we hypothesized that galactose cannot be utilized by yeast. Since each disaccharide molecule contains two subunits of monosaccharide monomers, yeast should decompose disaccharides into monosaccharides before going through glycolysis. Therefore, we hypothesized that yeast would utilize disaccharides in slower rate than it would utilize monosaccharides due to one more process that it needs in the overall process. Among disaccharides, the yeast would decompose sucrose and maltose in identical rates since the former is composed of glucose monomer and fructose monomer and the latter one is composed of two glucose monomer. We also hypothesized that yeast would decompose lactose in a slower rate since we expected that galactose, one of its two monomers, cannot be utilized in glycolysis.
Materials and Methods:
In this experiment, we studied the types of sugars that yeasts can utilize and the differences in rate that yeasts utilize sugars by applying fructose, galactose, sucrose, maltose, and lactose to five groups of yeast respectively. The positive control was set as the group of yeast that was treated with glucose and the negative control was set as the group of yeast that was only treated with buffer and pure water (Peck). Because carbon dioxide was a final product of the overall reaction, amount of carbon dioxide produced was proportional to the amount of sugar being consumed. Therefore, the rates that yeasts utilized sugars were indicated by amount of carbon dioxide being produced in a certain time period. We measured the amount of carbon dioxide produced by using a respirometer. The increase in volume of gas bubble in the respirometer indicated the volume of carbon dioxide produced. The rate that yeast utilized sugars were calculated by dividing volume of carbon dioxide produced (in milliliter) by time (in hour). This value was positively proportional to the rate that yeast utilized the sugar. We avoided radical change in pH value during the experiment by adding some kind of buffer solutions into all test tubes (Peck).
Results:
Figure 1: The relationship between the rates that yeasts utilized sugars and type of sugars added to yeasts. The rate is determined by change in volume of the bubble in respirometer divided by time that yeast contacted with sugar. The group that treated with yeast was the positive control in this experiment. The error bars represents standard errors.
The rates that yeasts utilized fructose and sucrose were statistically identical to the rate that it utilized glucose, because the differences in rates were less than one standard error (figure 1). The rate that yeast utilized maltose was a half of the rate that it utilized glucose. The changes in volume of gas bubble in respirometers of the group of lactose and galactose are statistically same to the change in negative control.
Discussion:
The yeasts could go through fermentation process with fructose, sucrose, and maltose, but could not go through fermentation with galactose and lactose, because the changes in volume of respirometer in groups treated with galactose and lactose were statistically same to change in volume of the negative control group, in which no fermentation would happen because no sugars were added. However, the rate that yeast utilized maltose was half of the rate that it utilized glucose.
The hypothesis about monosaccharides was proved to be correct because the yeast could not utilize galactose and utilized glucose and fructose in the same rate. But the experimental results regarding yeast utilization of disaccharides were different from the hypothesis because sucrose, which was expected to be utilized more slowly than monosaccharides, was utilized in the same rate as glucose was, and the yeast was not able to utilize lactose. We can conclude that yeast can utilize fructose, glucose, and sucrose in the same rate and utilize maltose at a half rate than it utilized the former sugars. It cannot undergo fermentation with galactose and lactose. The differences between result and hypothesis suggest that there are other factors determining if yeast can utilize sugars and the rate that yeast utilize them.
Since glycolysis is a process in which glucose is turned into pyruvate, the glucose is necessary for glycolysis to happen (Black et al. 158). Therefore, if the yeast doesn’t have enzymes required to convert other kinds of sugars to glucose, glycolysis cannot happen. Since yeast can utilize maltose and sucrose, it must have enzyme that can convert maltose and sucrose into glucose. Similarly, as yeast cannot utilize lactose or galactose, we can deduce that it lacks the enzyme necessary to convert lactose and galactose to glucose. However, fructose is an exception in this experiment. Fructose-6-phosphate, the phoshphyrated form of fructose, is part of glycolysis process (Black et al. 159). Hence the yeast can undergo glycolysis without converting it to glucose. The relatively low rate of utilizing maltose may be caused by low level of enzymes that convert maltose to glucose in yeast. If the yeast converts maltose to glucose more slowly than it converts sucrose, then the amount of glucose that can undergo glycolysis will be less, and the rate that yeast utilizes maltose will hence be consequently slow.
As glycolysis can only take place in cytoplasm, the sugar molecules must first enter the yeast cells (Black et al. 159). Since sugar molecules are polar, they are not likely to enter the cytoplasm through diffusion. Therefore, yeast should have proteins that help transfer certain kind of sugar molecules into its cytoplasm. It is possible that yeast doesn’t have specific type of protein that can help transport galactose and lactose into cytoplasm, and hence cannot undergo the process of glycolysis. The low rate that the yeast utilized maltose might be caused by the low rate that maltose is transported by the specific protein that transports the maltose.
Yeast fermentation is a crucial process of making wines and other alcoholic drinks, since ethanol is the ultimate product of this process. Studying the rate of yeast fermentation under different types of sugars can help us find out a more efficient way of producing ethanol. Therefore, results from this experiment may suggest the most efficient type of sugar that we can use to produce wines and other alcoholic drinks
Acknowledgement:
I would like to thank Professor Peck for helping me set up procedures of the experiment. My partners Maddy and Echo were very helpful during the experiment.
Works Cited:
1. Peck, Ron. “Glycolysis and Fermentation in Yeast.” Colby College, 2014. Print.
2. Black, Michael, Emily Taylor, Jon Monroe, Lizabeth Alliison, Greg Podgorski, and
Kim Quillin. Biological Science. 5th ed. Glenview: Pearson, 2014. Print.