Being a monosaccharide glucose is a single sugar unit which is very small and dissolves readily in water.
Sucrose and lactose are carbohydrates classed as disaccharides, they contain two sugar units. They are made when two monosaccharides join together by a reaction when one molecule of water is removed. This is known as a condensation reaction and produces a bond between the two molecules called a glycosidic link. The reaction will also go in the reverse direction, if a molecule of water is added to a disaccharide molecule two monosaccharide molecules are produced. This is hydrolysis and is what happens during the first stages of alcoholic fermentation with a yeast suspension.
Starch is a polysaccharide, a carbohydrate which is a polymer of monosaccharides. It is made up of many sugar units joined by condensation reactions. Since it is such a large molecule it is also insoluble.
Alcoholic fermentation with yeast suspension (water + yeast) uses glucose as a substrate, the overall equation being:
C H O CH CH OH CO
Therefore before fermentation can take place and the C O gas is produced and substrate which is not glucose must be broken down to glucose. This may take longer depending on the type of molecule being used as a substrate. Starch as a polysaccharide is the largest molecule and it will take the longest time for all the glycosidic links to be hydrolysed by the necessary starch specific enzyme to form glucose. The solution was only observed over a total of eight minutes, in the time the starch did not produce a result. However it can be assumed if the yeast contained the correct enzyme for hydrolysis and if it had been left for a longer period of time more of the glycosidic links would have had time to be hydrolysed and enough glucose would have been produced from the polymer to react with the yeast and a result would have been obtained.
The disaccharide molecules lactose and sucrose consist of glucose and fructose and galactose respectively. They therefore only have one glycosidic link that needs to by hydrolysed in order for glucose to be obtained and fermentation occur. If the enzyme necessary for hydrolysis of the glycosidic link is present fermentation can occur. If the enzyme is present it would therefore be expected for each disaccharide to produce a significant amount of carbon dioxide in the time given, although not as much as glucose itself
The enzyme that will hydrolyse the glycosidic links in sucrose is present in yeast, however no fermentation will occur with lactose as it does not contain the galactosidase (lactase) enzyme to hydrolyse the glycosidic link to form galactose and glucose.
Glucose as a monosaccharide is already in its monomer unit and does not need to undergo any hydrolysis before it reacts. It can therefore begin fermenting immediately and subsequently produces the greatest volume of gas within the timed period.
.
Fermentation is also an example of anaerobic respiration. In the absence of oxygen the Krebs cycle and the electron transport chain cannot function, only glycolysis takes place. This produces a little ATP: two molecules of ATP for each molecule of glucose and two pairs of hydrogen ions which must be removed if glycolysis is to continue. Fermentation occurs when these molecules are accepted by the pyruvate formed at the end of glycolysis, to give ethanol.
In alcoholic fermentation the pyruvate produced from the glycolysis is converted to ethanal as carbon dioxide is removed.
CH COCOOH CH CHO CO
Pyruvate ethanal carbon dioxide
The ethanal produced combines with hydrogen ions, transported by the hydrogen carrier NAD, to form ethanol.
NADH+ + H+ NAD+
CH CHO CH CH OH
Evaluating evidence and procedures
Sources of Error (in order of relative importance)
Temperature Control: A water-bath was used throughout but even with a thermometer and careful observation it was difficult to keep the constant throughout the experiment. An electric water-bath could have been used set at a specific temperature so that all the solutions were kept at the same temperature without fluctuations during the experiment. This may affect the results in that if the temperature was raised at any point the rate of reaction may have been faster for that particular experiment if the temperature dropped significantly the rate may have slowed. If the temperature became too high enzymes may also be de-natured.
Water in the glass tubing : It was very difficult to use the screws precisely so the water did not start in exactly the same place every time. This in turn meant the time that the gas took to reach the collecting tube differed. This could be improved by more appropriate and precise use of the screw clips.
Equipment : The equipment may not have been totally airtight, some gas may have escaped through some of the connecting parts which may have affected the volume of gas collected. Blue tack or Vaseline could have been used around connections to improve ensure no loss of gas.
The addition of yeast to the carbohydrate: The method used measuring the yeast suspension with a syringe meant it was difficult to displace the yeast from the syringe quickly. This also caused a delay in re-inserting the bung into the test-tube. Some gas may therefore have escaped and the timing delayed. This could be improved by injecting the syringe through the bung and using the screw clips appropriately.
Pooling of results: As collecting enough results in the time period allowed was difficult some results were pooled with others who used the same method. This may affect the results as practical procedures may have been slightly different. This could be improved by extending the time period or working faster to obtain more results.
There are no anomalous results in my data collected, (see graph enclosed). This is because none of the errors made were individual to one experiment, the conditions were constant throughout all experiments.
When the oxygen supply runs short in heavy or prolonged exercise, muscles obtain most of their energy from an anaerobic (without oxygen) process called glycolysis. Yeast cells obtain energy under anaerobic conditions using a very similar process called alcoholic fermentation. Glycolysis is the chemical breakdown of glucose to lactic acid. This process makes energy available for cell activity in the form of a high-energy phosphate compound known as adenosine triphosphate (ATP). Alcoholic fermentation is identical to glycolysis except for the final step (Fig. 1). In alcoholic fermentation, pyruvic acid is broken down into ethanol and carbon dioxide. Lactic acid from glycolysis produces a feeling of tiredness; the products of alcoholic fermentation have been used in baking and brewing for centuries.
Both alcoholic fermentation and glycolysis are anaerobic fermentation processes that begin with the sugar glucose. Glycolysis requires 11 enzymes which degrade glucose to lactic acid (Fig. 2). Alcoholic fermentation follows the same enzymatic pathway for the first 10 steps. The last enzyme of glycolysis, lactate dehydrogenase, is replaced by two enzymes in alcoholic fermentation. These two enzymes, pyruvate decarboxylase and alcoholic dehydrogenase, convert pyruvic acid into carbon dioxide and ethanol in alcoholic fermentation.
The most commonly accepted evolutionary scenario states that organisms first arose in an atmosphere lacking oxygen.1,2 Anaerobic fermentation is supposed to have evolved first and is considered the most ancient pathway for obtaining energy. There are several scientific difficulties, however, with considering fermentations as primitive energy harvesting mechanisms produced by time and chance.
First of all, it takes ATP energy to start the process that will only later generate a net gain in ATP. Two ATPs are put into the glycolytic pathway for priming the reactions, the expenditure of energy by conversion of ATP to ADP being required in the first and third steps of the pathway (Fig. 2). A total of four ATPs are obtained only later in the sequence, making a net gain of two ATPs for each molecule of glucose degraded. The net gain of two ATPs is not realized until the tenth enzyme in the series catalyzes phosphoenolpyruvate to ATP and pyruvic acid (pyruvate). This means that neither glycolysis nor alcoholic fermentation realizes any gain in energy (ATP) until the tenth enzymatic breakdown.