Biological Energy - Feeding Yeast!

On the last Sci-ber text page, you learned used to how iodine can used to be test for the presence of starch. Starch is a product made by plants from the sugar created by chloroplasts. Sugar is used by living things for energy. The process of converting sugar to mechanical energy always results in some energy being converted to heat.

In this experiment, you will use yeast. Yeast is a living organism that needs special treatment. Make sure that your yeast is fresh. The yeast that is not used should be refrigerated. Yeast cells consume sugar to obtain energy. As the yeast respires, it uses oxygen. The oxygen is converted to carbon dioxide.


  • Yeast
  • Sugar
  • 125 ml. Erlenmeyer flask
  • 4-inch balloon, deflated
  • Luke warm water
  • Masking tape
  • String
  • Metric ruler


  1. Measure the following and place each into the Erlenmeyer flask:
    • 20 g. of sugar
    • 120 ml. of lukewarm water.
  2. Measure 5 g. of yeast and place into the balloon.
  3. Quickly stretch the balloon over the opening of the Erlenmeyer flask.
  4. Seal the location where the balloon and flask meet using the masking tape.
  5. Lift the balloon upward and shake to release the yeast into the Erlenmeyer flask.
  6. Shake the Erlenmeyer flask and allow the reaction to occur.
  7. Measure the circumference of the balloon (using the string) every minute.
  8. Record each measurement.
  9. After each measurement, shake the flask gently to further mix the contents.
  10. Continue to measure and shake the flask until you have taken 20 minutes of data.

Safety concerns: Be sure to follow all glassware, and chemical safety rules that are specified by your teacher in all general laboratory experiences. DO NOT allow your beaker to be covered or stoppered. Make sure that you protect your body and clothing from the reactions. As with all science lab activities, the most important safety rule is to follow all teacher directions.

Record the data based on the time and circumference of the balloon.

Time (minutes) Circumference
Time (minutes) Circumference

Graph the data. Create a line graph using time for the X-axis and circumference for the Y-axis.


  1. What did you observe happening in this reaction?
  2. Where did the gas come from?
    • What gas do you think is being produced?
    • How could you prove what you think the gas is?
    • How do you think the gas was formed?
  3. What do you predict will happen to the balloon after 24 hours?

To observe how much heat energy is given off as the yeast consumes sugar, you will need to do a similar experiment using a digital thermometer. You will need to determine how you can cover the flask with the thermometer inside it the entire time. Measurements can still be taken each minute and recorded in a similar data table. A line graph would help you compare time and temperature.