The Effect of Substrate Concentrationon the Rate of Yeast Fermentation

Analysing Evidence

From the results tables it can be seen that the rate of fermentation increases as the concentration also increases. My results are presented on graph 1. Also, another graph can be drawn to present the class results (graph 2).

The graph which shows my results shows the rate of fermentation for each concentration. It clearly shows how the rate of fermentation (amount of carbon dioxide produced) increases as the concentrations get higher. For example, the trend line showing the 10% concentration increases very rapidly compared to the other 4 concentrations.

Both graphs show that the rate of fermentation increases as the concentration of glucose increases. “Fermentation is the breakdown of sugars by bacteria and yeast using a method of respiration without oxygen (anaerobic respiration). It involves a culture of yeast and a solution of sugar, producing ethanol and carbon dioxide with the aid of the enzymes.”

The rate of fermentation increases as the concentration increases. This is because the reaction rate depends on collisions between molecules. Molecules are in a state of a motion and therefore collide. The more collisions that take place the faster the rate of fermentation. If the concentration is high, the number of glucose molecules is abundant, therefore the probability that the enzymes (in yeast) come into contact with these molecules is high. This in turn increases the rate of fermentation. If the concentration is low, however, the number of glucose molecules is less and therefore the rate of fermentation decreases.

The conversion of glucose to carbon dioxide and ethanol takes place in the active site of the enzyme. Enzymes are protein chains of amino acids. Their molecules are very precise three-dimensional shapes. The shape includes a ‘dent’ which is exactly the right size and shape for a molecule of the enzyme’s substrate (in this case glucose) to fit into. This ‘dent’ is called the active site. When the glucose molecule slots into the active site, the enzyme changes it, pulling it out of shape and making it split into product molecules. The product molecules (in this case carbon dioxide and ethanol) then leave the active site, which is now ready to do the same to another glucose molecule. So if there are many glucose molecules (10% concentration) there will be more chance that these molecules can enter the active site and produce a fast rate of fermentation. This can also depend on the angle the collision between the glucose and enzyme takes place.  

The prediction I made earlier was that the rate of fermentation by yeast will be proportional to the concentration of sugar (glucose provided). I assumed that if I double the concentration of sugar the rate of fermentation would also double. From graph 2, at the 2.5% concentration the rate of fermentation is 0.18mm and if the concentration is doubled to 5.0% the rate of fermentation increases to 0.30mm. The rate of fermentation has more than doubled. If the concentration is increase to 10% the rate of fermentation becomes 2.21mm. This rate of fermentation has massively increased by over seven times.

My prediction has not worked and this could be due to several factors. The yeast solutions at the start of the experiment may not have been swirled thoroughly enough. This is important because many of the yeast cells may have remained at the bottom of the beaker so when we took our sample there would be an uneven balance of yeast. The temperature at which the experiment took place may not have been constant which would also affect the results.

Evaluation

The experiment went according to plan and sufficient results were gained to discuss my prediction. The results, however, were not very accurate because my prediction was proven incorrect. This is most likely to be because of the apparatus used. There are many problems that arose during the experiment. When the yeast begins to ferment some of the solution is pushed into the large test tube. This obscures the view of carbon dioxide produced and could cause errors in measurement. When carbon dioxide is produced some of it may dissolve into the solution and does not appear as gas, this again may cause errors. Human error also adds to the problem of inaccurate readings. As one of the products is ethanol, it can intoxicate some of the yeast cells which would die.  

Also, as fermentation takes place the sugar and yeast gets less and results produced may not be reliable. Therefore the procedure could have been altered in order to produce accurate results. During the experiment, there was a lack of control of other variables, such as temperature. Also, the yeast solution (before it is given to us) may not be well shaken, which leaves an unbalanced number of yeast cells.

The experiment could be improved in several ways. In order to control temperature, a water bath could be used to keep a constant temperature. The test tubes could be substituted for ones which have a scale down the side, in order to improve readings of carbon dioxide. Also, more sugar concentrations could be used to give a wider range of results. For example, the following concentration could be used instead: 0%, 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, 18% and 20%. This would produce more accurate results.