Category: My Classroom Material

Recreation of Pasteur’s Experiment

Introduction:

Today, we take many things in science for granted. Many experiments have been performed and much knowledge has been accumulated that people didn’t always know. For centuries, people based their beliefs on their interpretations of what they saw going on in the world around them without testing their ideas to determine the validity of these theories — in other words, they didn’t use the scientific method to arrive at answers to their questions. Rather, their conclusions were based on untested observations.

Among these ideas, for centuries, since at least the time of Aristotle (4th Century BC), people (including scientists) believed that simple living organisms could come into being by spontaneous generation. This was the idea that non-living objects can give rise to living organisms. It was common “knowledge” that simple organisms like worms, beetles, frogs, and salamanders could come from dust, mud, etc., and food left out, quickly “swarmed” with life. For example:

Observation: Every year in the spring, the Nile River flooded areas of Egypt along the river, leaving behind nutrient-rich mud that enabled the people to grow that year’s crop of food. However, along with the muddy soil, large numbers of frogs appeared that weren’t around in drier times. Conclusion: It was perfectly obvious to people back then that muddy soil gave rise to the frogs.

Objective:

In this experiment, you will conduct an experiment similar to the one done by Pasteur whenever he disproved spontaneous generation.

Materials Needed:

• Low-salt broth (chicken or beef, home-made or purchased)
• 2  250-mL Erlenmeyer flasks
• 2  1-hole rubber stoppers with bent glass tubing inserted (see diagram)
• Glycerine
• Hot plate & pot holders
• 50-ml Graduated Cylinder
• Marker

Procedure:

1. Students should work in teams of 2 to 3 people. Each team should perform the following steps.
2. Use glycerine and a twisting motion to insert glass tubing into the stoppers. be sure to rinse off excess glycerine with water.
3. Mark Erlenmeyer flasks accordingly:
   1. Flask 1 with stopper and glass tube going straight up
   2. Flask 2 with stopper and glass tube bent in S-curve
4. Using a graduated cylinder, place about 50-mL of broth in each Erlenmeyer flask.
5. Place appropriate lids on flasks.
6. Use a hot plate to boil broth in flasks with appropriate lids on them for 30 min., then let cool.
7. For the next ten days, observe the flasks and record any changes in color, turbidity, smell, etc. (Be careful to NOT remove the stoppers from the flasks.)

Data:

Conclusion:

1. What was the appearance on the broth in each flask on Day 1?
2. Was their an observed appearance change in flask 1 over the 10 days? Describe the change, if any.
3. Was their an observed appearance change in flask 2 over the 10 days? Describe the change, if any.
4. Explain why there was or was not a change in the appearance of the broth in each flask.
5. Why do you think the idea of spontaneous generation was believed to be true for so long (1000+ years)?
6. Did your experiment support spontaneous generation of organisms? Explain why or why not?