Are there any differences in the rate of metabolism of a dried yeast culture with differing carbohydrate sources? In the current practical that was undertaken the growth rate of yeast (S. cerevisiae) with differing carbohydrates sources : Glucose (C6H12O6), Fructose (C6H12O6), Lactose (C12H22O11), Xylitol “(CHOH)3(CH2OH)2” and Water (H2O) as a Control were observed. “ Yeast are single-celled fungi which consist of more than one thousand different species which have been identified. The most commonly used yeast is Saccharomyces Cerevisiae which has been utilized for the production of bread, wine and beer for thousands of years. Biologists in a wide variety of fields use S.Cerevisiae) as a model organism” (Simpson,2010). Fermentation is the metabolic procedure in which an organism alters a carbohydrate, such as sugar or starch, into an acid or alcohol. For instance , yeast performs fermentation to achieve energy by altering sugar into alcohol. Bacteria execute fermentation, altering carbohydrates into lactic acid (Helmenstine, 2013). Fermentation can be used in the food and drink industry as well, examples of this is bread. In the process of fermentation with bread, yeast produces carbon dioxide, alcohol and other compounds which enable the dough to rise and modify its physical properties.
Louis Pasteur stated that “ All yeast which ferment bread, beer, wine and cider corresponds to microscopic living cells of a microscopic fungus Saccharomyces cerevisiae . Many varieties of Saccharomyces cerevisiae exist in nature and they are more or less suitable for various types of fermentation” (Cofalec, 2013). The carbon source Glucose is basically sugar that provides the human anatomy with energy, the chemical comes from carbohydrates and in which the body alters into source of energy. Fructose is a monosaccharide which is mainly found in a lot of plants. Fructose is often bonded with glucose and together they form disaccharide sucrose. The organic compound Lactose is a disaccharide which comes from glucose and galactose which is commonly found in milk. Taken from crystalline alcohol resulting from xylose Xylitol can be used as an artificial sweetener in foods and can be found in some plant tissues. The objective of the experiment was to compare results between the four carbohydrate sources and their rate of metabolism. My hypothesis for the experiment is that the simplest sugar (monosaccharide) will be the best source of carbon. Method:
It is extremely crucial in the experiment to take health and safety precaution. These included, wearing gloves to not get any harmful substances onto the hands, wearing lab coats to prevent spillages pf chemicals onto an individual and also the use of forceps and other scientific equipment to ensure safety is obtained. To start off with, 0.1ml of the sample was taken using an automatic pipette from the flask and straight into the cuvette. It was important to use a fresh pipette tip for every time a new sample is used and assures accuracy. Then distilled water was added with a 0.9ml measurement and mix the two sample together, this was done by pipetting it up and down a couple of times. This process was repeated with all the sample with exactly 30 minutes intervals, this was measured by a stopwatch. 1ml of water and yeast solution was placed into a cuvette. This then needed to be placed into a spectrophotometer making sure the blank button is pressed, this was done to use a reference guide for the next set of findings. Lastly , the blank was removed and placed in the diluted sugar and yeast sample instead and the amount of absorbance was noted down
in a table. The independent variable of this experiment was the carbon sources, whereas the dependent variable was the growth rate and the absorbance of each substance. Throughout the experiment, the volume of solution, the temperature, the pH etc. all stayed the same; the only thing that wasn’t controlled is the carbon sources. Results:
Saccharomyces cerevisiae metabolised better with Glucose and Fructose because they are both monosaccharaides which means that both carbohydrates cannot be reduced by hydrolysis into another simple sugar. (Helmenstine, 2013) They can easily be broken down in the normal respiration pathway to release energy. This is called Glycolysis. Lactose is a disaccharide. If lactose is used for respiration, then it will have to be first hydrolysed to their monosaccharide components. Lactose is milk sugar, and most Saccharomyces cerevisiae cannot break it down, they don’t have the necessary enzyme to hydrolyse lactose to its monosaccharaides (J.F, 2013). However there is some respiration with lactose. With disaccharides they may have more enzymes to break which will take longer than monosaccharaides such as Glucose and Fructose. Saccharomyces cerevisiae does not naturally use xylose as a substrate, however, and must be engineered to both transport and ferment xylose (Jeffries, 2006). Native strains of Saccharomyces cerevisiae do not use xylose as a carbon source. However Wang et al. 1980, found out under intensive screening that some can convert xylose to ethanol directly under aerobic or oxygen limiting conditions, which is interesting to see (Jeffries and Jin, 2004). The Xylitol in the experiment did not react as well because there are 5 carbons so there is less substrate for the yeast to act upon and Xylitol is an isomer whereas Glucose and Fructose are better as they have 6 carbons and are monomers, which could be an evolutionary aspect. Distilled water (the control) was the lowest of them all as water does not contain carbon. It contains two hydrogen’s which are covalently bonded to an oxygen atom which is single. Figure 3: Structure of carbon sources.
Top Left: Glucose (Ophardt, 2003) Top Right: Fructose (Ophardt, 2003) Bottom Left: Xylitol (Helmenstine, 2013) Bottom Right: Lactose (Zamora, 2013).
Looking back at the hypothesis and comparing them to the trends in the data, I can see that it supports it as the simplest sugars, Glucose and Fructose reacted the highest. Therefore my hypothesis is accepted. Aspects of the experiment that went well is that health and safety precautions were in place, the volume of concentration for substances were measured very precisely and carefully and also the results matched the hypothesis. However if the experiment was done again then improvements would have to be made such as not leaving the carbon sources out of the spectrophotometer when they are not being used, to ensure accurate results. Also one of the spectrophotometers was not responding so this might have affected the results as the carbon sources were still in there.