example of discussion: We expected to reject our first null hypothesis and support the alternative that the rate of production of carbon dioxide by glucose is faster than that of fructose.
This is because glucose is the primary input for glycolysis, the first step in fermentation and aerobic oxidation; whereas fructose has to be converted to a useable form before it enters the process as an intermediate. Our expectation was also based on considerable literature that suggests that yeast has a higher affinity for glucose than fructose (Cason and Reid 10 1987, D’Amore et al. 1989). Briefly, notice for Figure 4, that we used two replicates for glucose as opposed to three as stated in methods and used for all other analyses.
This is because we ignored our results for the third glucose replicate as no CO2 was produced at all over the entire observed time period. We considered two sources of error for this anomaly. Firstly, that there were no cells present in the tube, which is unlikely considering the other replicates most definitely had live cells present. Alternatively, we noticed while recording results that there was an unusual air bubble in the third replicate tube that looked to be preventing the movement of liquid out of the tube, thus artificially returning results of zero CO2 production when in fact fermentation and oxidative phosphorylation were occurring. As n=2 for glucose, we cannot perform statistical analysis on this data. Unfortunately therefore, we cannot say anything statistically significant about our glucose and fructose results.
We can however recognize the very similar trend in carbon dioxide production between the two sugars that strongly indicates that we should fail to reject our first null hypothesis. After further investigation of sugar transport across yeast membranes, it became clear that this was in fact a reasonable result. Sugars do not freely permeate biological membranes (Lagunas 1993) therefore in order for sugars to enter the organism and be metabolized they must first cross the membrane via permeases, or transporters. Multiple studies report that fructose and glucose share the same membrane carrier (D’Amore et al. 1989, Lagunas 1993) thus when yeast are in media containing either glucose or fructose, as in the case of our comparative monosaccharide study, there should be no significant difference in rate of carbon dioxide production between the two as there is no competition for the carrier (D’Amore et al.
1989, Lagunas 1993). The 11 difference in affinity becomes important when glucose and fructose are in the same media, in which case the yeast preferentially uptake glucose (D’Amore et al. 1989). If we were to repeat our experiments we would be able to eliminate a considerable source of error we experienced, thus adding increased confidence to our results.
The source of error arose via the considerable amount of time that elapsed between resuspending yeast cells in media after centrifugation and getting the mixtures into the water bath to record CO2 produced. We recorded this to be a period of 23 minutes and 45 seconds. We performed secondary cell counts and equalized cell densities during this time, a necessary, but time consuming step. This contributed error as yeast cells will commence metabolic processes immediately after being re-suspended in media.
Important trends could have been missed during this time, particularly as we do not know whether fermentation and oxidative phosphorylation in Saccharomyces cerevisiae occur at a linear rate. As an extension of our first null hypothesis, we expected maltose, a disaccharide composed of two glucose residues, to produce carbon dioxide faster than sucrose, a disaccharide made up of one glucose and one fructose residue (De La Fuente and Sols 1962). Figure 4 clearly shows that this is not the case, thus we reject the second null hypothesis and support the alternative hypothesis that the rate of carbon dioxide production with sucrose is faster than the rate of carbon dioxide production with maltose in Saccharomyces cerevisiae. In our experiment, maltose did not result in any carbon dioxide production.
This was surprising to us as there are multiple accounts of maltose metabolism in wild type Saccharomyces in the literature (Cason and Reid 1987, D’Amore et al. 1989, Lagunas 1993). wen ?2018?8?14??? ??6:28???
