Apart from substrate concentration, which I will be changing in this experiment, it is important to keep the other variables the same to ensure the results are reliable and to get a fair test – results must change because of a difference in the input variable, not because of a different variable. I can monitor the temperature and check it stays the same by using a thermometer. I can keep the pH the same by always using water from the same source.
To keep the concentration of yeast the same each time the experiment is repeated I will accurately mix the same concentration each time. I will change the concentration of glucose every repeat of the experiment, and I will measure the output of gas each time. I did two repeats of a preliminary experiment for a 10% glucose concentration solution. This was to make sure the experiment planned was fair, and to sort out any problems found before carrying out the actual experiment.
Every variable apart from the glucose concentration will need to be kept the same, so this initial experiment is needed to check that the values of theseother variables aren’t interfering pr affecting the experiment. Some other things I needed to find out from this preliminary experiment are exactly how to measure the amount of oxygen produced and what temperature all the water used in the experiment should be. This is my first result: Glucose concentration (%) : 10 Amount of gas produced in 2 minutes (cm? ) : 0
This experiment didn’t produce a result because I used water at room temperature (20°C). Yeast didn’t respire rapidly enough to produce a result at this low temperature. For my second repeat experiment, I used the same concentration of glucose but I increased the water temperature to 30°C. This is my second result: Glucose concentration (%) : 10 Amount of gas produced in 2 minutes (cm? ) : 3 There are two things that could still be improved in this experiment. I started the stopwatch when the first bubble appeared, but to do so will not measure rate of respiration well.
This is because some of this early gas may be air from inside the boiling tube and the gas that is carbon dioxide will be given off unevenly because the yeast will not yet be respiring at a constant rate. Also very little gas was produced in 2 minutes, so I am going to wait 5 minutes for the rate of respiration to become a constant rate, then I am going to time how much carbon dioxide is given off by the yeast respiring for the 5 minutes after that. I am going to use water at 35°C.