Metabolism- Cellular Respiration
Laboratory Investigation


Upon completion of this lab, you should be able to:

• recognize the importance of respiration as a source of energy and gas
• determine which of several sugar sources is the most effective fuel for yeast
• determine how to measure the relative rate of CO2 production


Cells require energy to do their work, whether they are aerobic or anaerobic in their respiratory requirements. The reactions within cells that use energy stored in food molecules (such as glucose) to synthesize ATP are referred to as cellular respiration. Aerobic cellular respiration requires oxygen as the final electron acceptor. Fermentation does not require oxygen and uses a different molecule as the final electron acceptor. The equation for aerobic respiration is below.

C6H12O6 + 6O2 —> 6CO2 + 6 H2O + 36 ATP (38 ATP in prokaryotes)

In aerobic respiration, glucose is completely broken down to CO2 + H2O, but during fermentation, it is broken down only part-way. Much of the energy originally available in glucose remains in the products produced. Plant and fungal cells produce alcohol as a result of fermentation, and animal cells produce lactic acid. The equation for alcohol fermentation is below.

C6H12O6 —> 2CO2 + 2C2H5OH (ethyl alcohol) + 2 ATP

It should be clear from the above equations that aerobic respiration produces much more ATP per glucose molecule than fermentation. The more completely the original fuel molecule is degraded, the more energy will be released.


Yeast as a Test Organism

Yeast is a convenient source of eukaryotic cells. These cells are unusual in that they can survive long durations of dehydration and redevelop living qualities when again hydrated. Yeast cells are also unusual in that they can survive anaerobic conditions by producing ethanol as a waste product. Under aerobic conditions, the yeast break down glucose using oxygen and get more ATP, as in the reaction above. (Also in the presence of oxygen, yeast can utilize their anaerobic waste product of ethanol to produce ATP by sending it back into the Krebs cycle via acetyl CoA.)

In this experiment you will test the ability of yeast to utilize a number of food sources. Relative gas production will be measured and compared for each source. The gas being measured is carbon dioxide. The rate of cellular respiration is proportional to the amount of CO2 produced (see the equation for fermentation above). In this experiment, we will measure the relative rate of cellular respiration using several different food sources.
Go to this link and follow the procedures shown. It will serve as both a review and a means of thinking about the investigation you will be doing.


Materials Required

5 packets of brewer’s yeast
7 plastic soda or water bottles (.5 liter or smaller)
   [They should all be the same size. If soda bottles are used, they must be washed thoroughly so that no sugar residue is left in the bottle.]
Thermometer, marker pen
Balloons, tape measure, measuring cups
Distilled or bottled water (700 ml ~ 3 cups)
Individual packets of Equal©, Splenda©, Sweet ‘n Low©, Plain Sugar
Small bottles of grape juice, sweetened and unsweetened
Small kettle for heating water,serving bowl that will hold at least 3 cups of water
Spoon for stirring, Saran Wrap or aluminum foil


  1. Label each of 7 bottles with the marker. Bottle 1 is Control, 2 is Sugar, 3 Equal, 4 Splenda, 5 Sweet ‘n Low, 6 sweetened grape juice, 7 unsweetened grape juice. Into bottle 2 put 1 packet of sugar, into 3 a packet of Equal, into 4 a packet of Splenda, into 5 a packet of Sweet ‘n Low , into 6 a quarter-cup of sweetened grape juice, and into 7 a quarter-cup of unsweetened grape juice. Be sure to rinse out the measuring cup before putting in the unsweetened grape juice.
  2. Open 5 packets of brewer’s yeast and pour them into a serving bowl. Add 3 cups of distilled or bottled water and mix thoroughly. Remember that this is a suspension, not a solution, because the yeast does not dissolve. The yeast will settle to the bottom, so the suspension will need to be stirred before it is distributed into the various bottles. You now have about 700 ml of yeast suspension.
  3. Heat some water in the kettle and heat it to 35º C. Place the bowl with the yeast suspension into the kettle and leave it there for 10 minutes. Do not allow the temperature to rise above 40º C, since that will kill the yeast.
  4. Cover the bowl with the yeast suspension with Saran Wrap or aluminum foil while it is being heated.
  5. At the end of the 10 minute heating period, remove the yeast suspension and distribute an equal amount of the suspension (about 100 ml) into each of the labeled bottles.
  6. Place a balloon over the end of each bottle. Be sue that the balloon fits snugly on the neck of the bottle.
  7. Sit back and wait. Record the time and bottle number of the balloon that inflates first. After a balloon begins to inflate, measure the circumference every fifteen minutes for one hour and record your data in a data table. Allow the setups to sit undisturbed for 24 hours and once again record the circumference of each of the balloons. Wait another 24 hours and measure and record your data again.

Kitchen conversions

Site 1

Site 2

Site 3

Question 1

Construct a graph showing the change in size of the balloon over time for each of the bottles. Submit the graph and your data set to the instructor via email at

Question 2

Considering the results of this experiment, do yeast utilize all sugars equally? Explain.

Question 3

Hypothesize why some sugars were not metabolized while other sugars were.

Question 4

Why do you need to incubate the yeast before you start collecting data?

Question 5

Yeast live in many different environments. Why is the human body an ideal place?