Problem:

Yeasts undergo aerobic cell respiration if there is sufficient oxygen and releases carbon dioxide as a waste product. Yeasts, like any other cells, have an optimum temperature at which they work most efficiently, including the process of cell respiration. This experiment aims to discover the relation between temperature and the carbon dioxide yield of yeasts to discover the optimum temperature for yeasts’ execution of aerobic cell respiration.

Hypothesis:

It is hypothesized that yeasts carry out aerobic cell respiration most efficiently at high temperatures because high temperature is likely to activate the process in a higher rate. Cells are most active in high temperatures yet within their tolerance of heat, if temperature exceeds 40 degrees, yeasts, along with their enzymes, will die off or become denatured so they no longer function. On the contrary, low temperature will not activate the yeasts to work as yeasts are not adapted to cold environment.

Variables:

  • Independent variable: temperatures of 10% glucose solution in which yeasts are placed to carry out aerobic cell respiration (6°C, room temperature, and 30°C are the temperatures investigated, though the actual room temperature at the lab is noted down)
  • Dependent variable: Change in CO2 concentration after yeasts were placed in the glucose solution over time at different temperatures (CO2 concentration in 3 minutes recorded at an interval of 30 seconds)
  • Constants/controlled variables: concentration of glucose solution (10%), mass of glucose solution used at each trial (50 gm of water and 5 gm of glucose), mass of yeasts used at each trial (250 mg), rate of stirring of solution on the stirring plate (500 rpm), time to record CO2 concentration after yeasts are put in (30 seconds, 1 minute, 90 seconds, 2 minutes, 150 seconds, 3 minutes), chemicals used (10% glucose solution), apparatus and equipment (test tubes, 100 ml beakers, 50 ml graduated cylinders with uncertainty of ±0.1 ml, 250 ml Erlenmeyer flasks, balance in g accurate to 2 decimal places, hot plate that also contains a magnetic stirrer plate and magnetic stirring bar, thermometer range from 0°C to 100°C with an uncertainty of ±0.01°C, CO2 sensor which connects to an Xplorer GLX machine, test tube racks, timer accurate to 0.01 s, 1 spatula, 1 ice bath consists of a 50 ml beaker and ice cubes, 1 fridge)

Materials:

  • Yeasts 1.5 g
  • Glucose 30 g
  • 500 ml distilled water
  • 4 100 ml beakers, uncertainty ±5 ml
  • 1 thermometer ranged from 0°C to 100°C with an uncertainty of ±0.01°C
  • 12 test tubes
  • 1 test tube rack that can hold 12 test tubes
  • 1 50 ml graduated cylinder with uncertainty of ±0.1 ml
  • 6 250 ml Erlenmeyer flasks, uncertainty is not concerned as they are not used to measure volumes
  • 1 Weighing balance in g accurate to 2 decimal places
  • 1 spatula
  • 1 hot plate that also contains a magnetic stirrer plate
  • 1 magnetic stirring bar
  • 1 CO2 sensor charged to full battery with a stopper binds to the flask
  • 1 GLX machine with full battery
  • 1 timer accurate to 0.01 s
  • 1 ice bath, including 1 50 ml beaker and 6 ice cubes with a side length of approximately 1 centimeter
  • 1 fridge with a refrigerator compartment that refrigerates at a temperature higher than 0°C, approximately 0 to 4°C

Procedures:

6 °C glucose solution preparation:

  1. 100 ml beaker is filled with distilled water
  2. 6 ice cubes with side length of approximately 1 centimeter are placed in the 100 ml beaker with distilled water
  3. The 100 ml beaker is laid aside in the refrigerator compartment with a temperature ranged from 0°C to 4°C but higher than 0°C so water will not freeze in the fridge over night
  4. On the second day, 5 g of glucose is weighed on a weighing balance accurate to 2 decimal places
  5. 5 g glucose is put into a test tube and placed on the test tube rack
  6. Similarly, 0.25 grams of yeasts are measured by a weighing balance accurate to 2 decimal places and transferred into a test tube, which is placed on the test tube rack
  7. The 100 ml beaker stored in the fridge is taken out and poured into a 50 ml graduated cylinder with uncertainty of ±0.1 ml to measure 50 ml of ice water, ice cubes will stay in the beaker
  8. 50 ml ice water is poured into the 250 ml Erlenmeyer flask
  9. The Erlenmeyer flask is placed on a hot plate which also functions as a magnetic stirring plate and a magnetic stirring bar is put into the flask
  10. 5 gram of glucose already measured in the test tube is poured into the water
  11. The stirring plate is turned on, stirring at a rate of 500 rpm
  12. The stirring plate is turned off once glucose is fully dissolved
  13. A thermometer ranged from 0°C to 100°C with an uncertainty of ±0.01°C is inserted into the glucose solution, at this stage, the ice water is warmed up, the procedure cannot be proceeded until the temperature of the solution reaches 6°C
  14. The CO2 sensor is connected to the GLX machine which displays the CO2 concentration in the air
  15. The CO2 sensor, which attached to a stopper that binds to the neck of the flask to block flow of air, is imbedded into the flask
  16. The CO2 concentration will be displayed on the GLX machine and is noted as the original CO2 concentration in the flask
  17. The CO2 sensor is pulled out and yeasts in the test tube is poured into the flask, CO2 sensor is put back into the flask, magnetic stirring plate is turned on at a revolution rate of 500 rpm, stopwatch is ticked off, all this should be done without intervals
  18. The CO2 concentration in the flask is recorded every 30 seconds for 3 minutes so 6 numbers will be recorded
  19. The procedure above is repeated twice more

Room temperature glucose solution preparation:

  1. 100 ml beaker is filled with distilled water
  2. The beaker is placed in the lab over night
  3. On the second day, 5 g of glucose is weighed on a weighing balance accurate to 2 decimal places
  4. 5 g glucose is put into a test tube and placed on the test tube rack
  5. 25 grams of yeasts are weighed on a balance accurate to 2 decimal places
  6. 25 g yeasts are transferred to a test tube and placed on the test tube rack
  7. The distilled water in the 100 ml beaker is poured into a 50 ml graduated cylinder with uncertainty of ±1 ml to measure 50 ml of water at room temperature
  8. 50 ml water is poured into the 250 ml Erlenmeyer flask
  9. The Erlenmeyer flask is placed on a hot plate which also functions as a magnetic stirring plate and a magnetic stirring bar is put into the flask
  10. 5 grams of glucose already measured in the test tube is poured into the water
  11. The stirring plate is turned on, stirring at a rate of 500 rpm
  12. The stirring plate is turned off once glucose is fully dissolved
  13. A thermometer ranged from 0°C to 100°C with an uncertainty of ±0.01°C is inserted into the glucose solution, the temperature measured should be the room temperature and is noted for further examination
  14. Steps 14 to 19 in 6°C glucose solution preparation from last section are repeated

30°C glucose solution preparation

  1. 50 ml distilled water is measured with 50 ml graduated cylinder with uncertainty of ±0.1 ml
  2. 50 distilled water is transferred to the 250 ml Erlenmeyer flask
  3. 5 g of glucose is weighed on a weighing balance accurate to 2 decimal places
  4. 5 g glucose is put into a test tube and placed on the test tube rack
  5. 25 g of yeasts are measured by a weighing balance accurate to 2 decimal places
  6. The yeasts are put into a test tube and placed on the test tube rack
  7. 5 gram of glucose already measured in the test tube is poured into the water in the flask
  8. The flask is placed on the top of hot plate that also functions at a magnetic stirring plate, which is set to 30 degrees and turned on the stirring at a rate of 500 rpm
  9. The stirring plate is turned off once glucose is fully dissolved
  10. A thermometer ranged from 0°C to 100°C with an uncertainty of ±0.01°C is inserted into the glucose solution to monitor the change in temperature
  11. Once the temperature reaches 30°C, the thermometer is taken out, hot plate turned off
  12. Steps 14 to 19 in 6°C glucose solution preparation from the second last section are repeated

Methods of control of variables:

The independent variables are temperatures of 10% glucose solution in which yeasts are placed to carry out aerobic cell respiration. They are 6°C, room temperature, and 30°C respectively. The methods of how to manipulate the independent variables are explained in the procedure. Briefly, 6°C glucose solution needs to have a distilled water stored in an ice bath and placed in the refrigerator compartment of a fridge. Note that it cannot be put in to the freezer compartment, otherwise the distilled water will be frozen so cannot be used. Overnight, the temperature will be close to the temperature of the refrigerator compartment, which should be between 0 to 4°C. The water will be taken out the second day and its temperature is measured with a thermometer ranged from 0°C to 100°C with an uncertainty of ±0.01°C. The temperature is likely to rise during the process, so once the temperature reaches 6°C, the rest of the procedure can be carried out. The room temperature glucose solution requires a similar set up as the 6°C solution. Distilled water fills the 100ml beaker and is placed in the lab overnight to require water at a room temperature. The exact temperature however, should be noted for quantitative analysis. The 30°C glucose solution requires a hot plate. Distilled water is heat up on the hot plate set at 30°C and a thermometer is inserted into the flask with distilled water to monitor change in temperature. Once the temperature reaches 30°C, the hot plate is turned off and the rest of the procedure must be carried out immediately so that the water or the glucose solution does not cool down.

The dependent variable is the change in CO2 concentration after yeasts were placed in the glucose solution over time at different temperatures. One would record the initial CO2 concentration of the glucose solution before yeasts were put in to obtain the stock concentration of CO2 in the flask. Then, the CO2 concentration in the flask is recorded at a 30 seconds interval after yeasts are put in for 3 minutes, so after 30 seconds, 1 minute, 90 seconds, 2 minutes, 150 seconds and 3 minutes the CO­2 concentration are recorded. One would need a timer for this. The experimenter may wish the record the data and graph them in a graph of CO2 concentration in the flask over time. With the graph, one would subtract the initial CO2 concentration from the CO­2 concentration, the CO­2 concentration at 30 seconds from the CO2 concentration at 1 minute and so on the obtain the difference of CO2 concentration between each interval to monitor the overall rate of change in CO2 concentration at different temperatures in which yeasts carry out aerobic cell respiration.

One controlled variable is the concentration of glucose solution, which is kept at 10% by measuring 5 grams of glucose with a weighing balance accurate to 2 decimal places and then the glucose is poured into a 50 ml distilled water measured by a 50 ml graduated cylinder with an uncertainty of ±0.1 ml, mixed by the magnetic stirring plate and bar. This process is carried out in all three temperatures.

Another constant is the mass of glucose solution used at each trial. Similar to the previous one, 50 gm of distilled water are measured by a 50 ml graduated cylinder and 5 gm of glucose are weighed by a weighing balance. They will be mixed using a magnetic stirring plate and a magnetic stirring bar put into the Erlenmeyer flask that contains the distilled water and glucose. Note that the glucose can be stored in a test tube and put aside and when being poured into the flask containing distilled water, the test tube is knocked lightly to ensure glucose would not stick to the internal surface of the test tube so most if not all 5 gm of glucose will go into the flask.

The mass of yeasts used at each trial – 250 mg – is another controlled variable. Like glucose, yeasts are also measured with a weighing balance accurate to 2 decimal places in grams. Weighing yeasts and transferring them into a test tube can be tricky, extra concentration is required.

The rate of stirring of solution on the stirring plate is 500 revolution per minute. The indicator should be switched to 500 rpm with the magnetic bar placed inside the solution and with yeasts, if numbers of rpm are not shown, revolution rate is switched to medium instead.

Another constant is the time to record CO concentration after yeasts are put in. The time are 30 seconds, 1 minute, 90 seconds, 2 minutes, 150 seconds, 3 minutes. A stopwatch or a timer is required and also accurate to 0.01 s. At these 30 seconds intervals, the CO­2 concentration is displayed on the GLX machine connected to the CO2 sensor, the number is jogged down by looking at the number displayed in ppm.

The chemicals used, i.e. 10% glucose solution, is another controlled variable. Remember only glucose, not other sugars, are investigated in this experiment. The apparatus and equipment used are constants as well. They are 12 test tubes, 100 ml beakers, 50 ml graduated cylinders with uncertainty of ±0.1 ml, 250 ml Erlenmeyer flasks, balance in g accurate to 2 decimal places, hot plate that also contains a magnetic stirrer plate and magnetic stirring bar, thermometer range from 0°C to 100°C with an uncertainty of ±0.01°C, CO2 sensor which connects to an Xplorer GLX machine, test tube racks, timer accurate to 0.01 s, 1 spatula, 1 ice bath consists of a 50 ml beaker and ice cubes, 1 fridge. The numbers of each equipment are listed in the materials section. New equipment can be used if the previous one has residue of solution, e.g. glucose solution with yeasts, so the procedure can be carried in a faster fashion.

Equations or methods to collect relevant data:

Data table 1:

Temperatures (°C)TrialInitial CO2 concentration (ppm)CO2 concentration after 30 seconds (ppm)CO2 concentration after 60 seconds (ppm)CO2 concentration after 90 seconds (ppm)CO2 concentration after 120 seconds (ppm)CO2 concentration after 150 seconds (ppm)CO2 concentration after 180 seconds (ppm)
61
2
3
n (room temperature)1
2
3
301
2
3

 

Data table 2:

Temperatures (°C)TrialChange in CO2 concentration from initial concentration to after 3 minutes (ppm)CO2 production per second per gram of yeasts (ppm/s/g)Average CO2 production per second per gram of yeasts at each temperature (ppm/s/g)
61
2
3
n (room temperature)1
2
3
301
2
3

 

This experiment aims to investigate the relation between the temperature of the solution as an environment in which yeasts carry out aerobic respiration and the carbon dioxide production by yeasts. The correlation is whether positive or negative yet the optimum temperature for yeasts to carry out aerobic respiration cannot be determined by only investigating 3 temperatures. However, we can infer the property of the correlation by examining and comparing the rates of change in CO2 concentration at each temperature of the glucose solution.

First, the temperature of the glucose solution placed in the lab overnight which should be equivalent of the room temperature is measured with a thermometer ranged from 0 to 100°C with uncertainty of ±0.01°C. Then the n value in the first column on both data tables can be filled.

Second, the initial concentration of carbon dioxide in the flask with 10% glucose concentration is recorded using a CO2 sensor that is inserted into the flask and connected to a GLX device that displays the concentration. This stock concentration acts as a reference to how much carbon dioxide present in the flask in the first place. This concentration is measured in each trial because the CO2 concentration is likely to fluctuate in each trial and will influence the results of CO2 concentration measured later.

Third, after yeasts were put into the flask with glucose solution, the CO2 sensor is put back on and timer ticked off. At each interval of 30 seconds, the CO2 concentration is recorded. If the new concentrations rise, it indicates the yeasts are carrying out aerobic cell respiration in which they breathe in oxygen and release carbon dioxide as waste product. Because there is abundant oxygen gas present in the flask beforehand, the yeasts are expected to continuously carry out aerobic respiration through the 3-minute trial. On the contrary, if the CO2 concentration remain the same or even starts to decrease, it implies that yeasts are not functioning. It is either because the temperature does not fulfil the requirement of yeasts to work or the yeasts start to die off as enzymes denatured from high temperature. The CO2 concentration measured is recorded at 30 seconds, 60 seconds, 90 seconds, 120 seconds, 150 seconds and 180 seconds after yeasts are placed into the glucose solution with CO2 sensor attached stopper sealed up the neck of the Erlenmeyer flask so air cannot flow. The data acquired should fill the entire data table 1.

Data table 1 consists of raw data only. And data table 2 is filled with processed data.

Fourth, after all raw data are obtained, processed data can be calculated. The change of CO2 concentration in the flask from the initial concentration to the concentration after 180 seconds since yeasts are put in can be deduced by deducted the stock concentration from the carbon dioxide concentration in the flask after 180 seconds since yeasts are put in the glucose solution. This change of concentration leads to the first speculation of the efficiency of yeasts under these temperatures. The equation is therefore used:

concentration of CO2 in the flask after yeasts are put in for 180 seconds in ppm – concentration

of initial CO2 in the flask in ppm = change in CO2 ­concentration

Fifth, the CO2 production per seconds per gram of yeasts can therefore be calculated by divide the change in CO2 concentration with 180 seconds and then divide again by 0.25 g of yeasts that had been used. The overall three sets of data representing three trials under different temperatures will be added together and divide by three to determine the average rate of change in CO2 concentration. The following formula is used:

Average production of CO2 per second per gram of yeast in ppm/s/g = (change in CO2 ­concentration in first trial in ppm/180 s/0.25 g + change in CO2 ­concentration in second trial in ppm/180 s/0.25 g + change in CO2 ­concentration in third trial in ppm/180 s/0.25 g)/3

Once the average rate is calculated in ppm/s/g, the three rates can be compared to determine under which of the three temperatures is the most efficient for yeasts to carry out aerobic cell respiration. The highest one would mean that the related temperature is likely to be closer to the optimum temperature at which yeasts function most efficiently because more carbon dioxide produced per second per gram of yeasts under one temperature means the temperature is more favored by yeasts among the three temperatures examined.

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