Experiment to Investigate Factors Affecting the Rate of Respiration in Yeast

Preliminary Objectives

• Find out what is the optimum, maximum and minimum temperatures for the respiration of yeast.
• Which is the best surface area/mass for the yeast
• Best % glucose solution to be used for the experiment
• Find out the most useful settling time

Finding Out the optimum, maximum and minimum temperatures that yeast can survive at:

I set up a simple experiment to do this.  I heated some water in a kettle, which turned out to be around 80°C, which I knew was too high, so I added cool water until the temperature was at 60°C. I then put 1 gram of yeast into a test tube with 10cm3 of 12% glucose solution inside.  

This showed us that in the temperature 60°C, the yeast could still respire, but quite slowly compared with another experiment I set up at 40°.  I decided to try a 40°C experiment because that particular temperature was about half of the boiled water’s temperature coming out of the kettle.  This produced a quicker rate of respiration that the 60°C experiment.  I also tried a 20°C experiment, but this had a lower rate of respiration than the 40°C experiment.  I concluded from this that 40°C must be around the optimum temperature, while 20° and 60°C must be around the minimum and maximum temperatures; as they are both 20° from the temperatures I definitely know will kill yeast, 0° and 80°.

Which is the best surface area for the yeast?

I decided that I would use a 1 gram ball of yeast, as this would mean that no matter how many shapes I tried the sphere would have the largest surface area. This is because whereas the cube has corners on it, which do not have a surface area, the sphere uses the spaces where the corners would be to provide a larger surface area.  I wanted a large surface area so there would be increased absorption of glucose into the yeast.  I decided on just one gram of yeast because I realised that any more would mean that it would not be submerged properly in the small test tube, and this would mean the test would be unfair.  

The best % of glucose to be used:

I knew from research that I had conducted that the higher the % of glucose solution, the quicker the reaction.  This is basically because there is more glucose to fuel the reaction. I chose to use a 12% solution, as this was the highest % of solution I had access to.

The most useful settling time:

To find out the most useful settling time, I found the average settling time of the three preliminary experiments I carried out.  These were 20°C, 40°C and 60°C.  Obviously, the higher the temperature, the longer the settling time was.  The average settling time was about 3 and half minutes.  I chose to keep this constant throughout the experiment, as much as I could, and to add cool water to the higher temperature experiments in case this settling time was too short.  

Plan     

I will set up equipment as shown in the diagram below, to collect a preliminary set of results:

I measured out 10cm3 of 12% glucose solution, and poured this into a 25cm3 test tube.  Both components were at room temperature.  Then I placed the test tube in the water bath as shown above, and waited until the 3 minutes allocated settling time was up, before introducing the 1g ball of yeast into the glucose environment. I then sealed the bung over the top of the test tube. I would then record the volume of gas displaced every minute until the yeast stopped respiring.  This would indicate how long I should measure the volume of gas displaced for.

 Following this, I had proposed to wait to see the change in the volume of gas in the displacement chamber, however, upon reaching 16 minutes I discovered that I had just recorded 5 of exactly the same result, 18ml.  I then realised that this must mean that the yeast had stopped respiring.  The following results were recorded:

I can see from the results and graph on the previous page, that the trend was that the longer we left the yeast, the higher the volume of CO2 produced.  This trend occurred from 4 minutes until 16 minutes.  I can assume from these results that the best time to leave the experiment for would be 15 minutes, in case there would be any change after 12 minutes in another environment.  

Actual Method

Fair Test:

                The following measures will be kept constant to make sure that this investigation is fair:

• The % of Glucose concentration will be kept constant, to avoid a fluctuation in results
• There will always be 10cm3 of solution in the test tube
• A 1g ball of yeast will be used, and changed each experiment
• An accurate pair of electronic scales will be used to avoid human error
• The test tube will be washed out after each experiment
• An accurate measuring cylinder will be used, because we are only measuring very slight changes in volume
• An accurate stopclock will be used (accurate to 100th/s) so our results will be as accurate as possible.

Diagram

Equipment list:

• Yeast
• 12% Glucose Solution
• 2 thermometers
• Water
• Kettle
• Beaker
• Test tube
• Rubber tube and bung
• Big water tub
• Stop clock
• Electronic scales
• 25cm3 measuring cylinder

Safety/Hygiene Precautions:

• Take care when handling hot water
• Be careful with glass test tubes, beakers and thermometers
• Wash hands after handling yeast or glucose solution
• Follow Lab Safety Rules

Prediction and Graph:

                                     I predict that as the temperature is raised towards the optimum, the graph will rise steadily as shown.  However, I also think that as the temperature passes the optimum, the graph line will return back to the line of origin in a similar fashion to that which it ascends by.  A graph to illustrate this is attached and titled ‘Predicted Graph’.

Method

I used the same principal as I used in the Plan earlier.  This involved setting up the experiment as shown in the diagram.  I filled a large water tub full of ordinary tap water, and then submerged a measuring cylinder under the surface.  This was so there was some water to be displaced by the CO2.  Then I carried out the following experiments: 20°C, 30°C, 40°C, 50°C and 60°C.  Each time I finished an experiment, I changed the water in the beaker, washed out the test tube thoroughly and boiled some new water.  Throughout my experiments I kept to the fair test guidelines I drew up earlier.  

Collecting Results

For each temperature, I conducted 3 experiments.  This was so I could see whether there were any anomalous results being recorded, and so that if there was an error in one of the experiments it would show up and I could repeat that particular test.  One problem I had with the preliminary results was that the differences between the results were so slight that there was really no point recording all 15.  This was why I decided to only record the starting and finishing volumes in the experiment.  I also calculated an average for each experiment, after anomalous results that did not fit the trend were repeated.  

Results and Graph (see previous page)

Anonymity within results:

There were certain anonymous results shown in the graph and table overleaf.  These were test numbers: 1 (20°C), 1 (30°C), 2 (40°C), 1 and 3 (50°C) and 1,2 and 3 (60°C).  The most likely explanation for most of these anonymous results would probably be ‘human error’.  Especially when there are very slight differences in results (e.g. Test One at 20°C) human error becomes apparent.  One common human error in this experiment would be an incorrect reading off the measuring cylinder, or some air escaping into the displacement chamber after a slip of the hand.  Unfortunately, I only realised that the 60°C experiment was TOTALLY anomalous after the apparatus was no longer available, and was unable to carry it out again.  The trend should have meant that the 60°C temperature should have produced a lower set of results than the 50°C one.  In estimation, I believe that the average for the 60°C experiment should have been something like 3ml.  The most likely error that contributed to this particular set of poor results would be that the water had cooled, therefore reducing the temperature towards the optimum level, and allowing the yeast to respire quickly, which is why the average is almost as high as the optimum temperature’s average.  

Analysis

Evaluation:    

In evaluation, I believe that my experiment would have worked well but for the anonymous 60°C result.   I followed the Fair Test guidelines throughout my experiments so as to get an accurate set of results, however I made a mistake, which cost me these results.  If I had realised this mistake sooner, then I would have rectified it by repeating this experiment again.  I followed my safety rules as well, and this meant that I didn’t cut or burn myself.  I followed my Plan and Method, and these ensured that my experiments ran smoothly and efficiently, and the minimum of time was wasted.  Where there were anomalous results I carried out the particular experiment again (excepting the 60°C experiment).  I also believe that I achieved my aim at the start of the coursework, and this is shown in the Conclusion.

Conclusion

 When I started this investigation, my aim was to investigate factors affecting the rate of respiration in yeast.  I think that I have achieved this.  I have found out that the rate of respiration in yeast is affected by a number of factors, and these include the factor I chose to investigate (temperature of the surroundings).  I found during my experiments that as I raised the temperature of the environment from 20°C through to 60°C, at first the rate of respiration within yeast rose, as shown in the graph.   Then, I believe that when the graph reaches its peak at around 40°C, this must be the optimum temperature for the respiration in yeast.  However, after this peak, I expected the graph to turn and follow a route back down, similar to the one it took whilst the rate of respiration increased, this did not happen.  Instead, possibly due to the cooling of some of the water, the graph leapt back up again, back up to the optimum temperature.  This could be because when the water started off at 60°C, and was left for the 15 minutes, it cooled, and as it cooled, it passed 40°C on the way down, suddenly letting the yeast respire at it’s full capacity.  Because we did not take results every minute, we failed to realise this until the experiment had finished, we had packed up our apparatus and it was no longer available.  Had we repeated the experiment with 60°C water used all the way through, I expect that the graph would look more like the predicted graph shown earlier.

  I believe that this experiment could have been improved by firstly repeating the 60°C experiment.  Then I could have improved accuracy by using a more sensitive electronic scale, and by having a proper displacement chamber, instead of the makeshift one we used.  A proper displacement chamber would have allowed us to measure the volume of gas produced in more accuracy than by reading it off a scale.  Another problem I had was that when I was reading the volumes from the displacement chamber, I had to bend down to the level of the chamber (about table height) to read the measurement off – whilst I was still holding the apparatus in place.  It would have been easier to do this and to avoid human error by using a clamp and retort stand to hold the measuring cylinders in place, instead of having to do this by hand.  

  However, under the circumstances, and without taking these improvement points into consideration, the experiment proved that the temperature of the environment that yeast is placed in does affect its rate of respiration.