WEB TUTORIAL 1.1
Science as a Process:
Arriving at Scientific Insights
Text Sections
Section 1.2 What Is Science?, p. 3
Introduction
Does life regularly arise from anything but life, or can it be generated “spontaneously,” through the coming together of basic chemicals? How can the issue be
decided? What are the fundamental rules of scientific experimentation? In this
tutorial, you'll examine two classic experiments on spontaneous generation and
review the use of experimental and control groups to test a hypothesis.
Learning Objectives
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Know the steps in the scientific method.
Understand the definition of the term “hypothesis.”
Discuss the hypothesis of spontaneous generation.
Narration
Spontaneous Generation
Where do cells come from? If a cut of meat is left out, over time it will putrify and
begin to teem with microorganisms, and possibly with larger organisms like maggots. Prior to the mid-to-late 19th century, the origin of microorganisms in decaying matter was in question. Some maintained that microbes arose from other
microbes that landed on the food from the air. Others supported the hypothesis of
spontaneous generation, which states that living organisms can arise spontaneously from nonliving matter.
Redi's Experiment
In the 1600s, Francesco Redi sought to test the hypothesis of spontaneous generation by applying what came to be known as the scientific method-a process of making observations, asking questions, formulating a hypothesis, and designing experiments to test the hypotheses.
Redi and others observed that flies and then maggots could be seen around pieces
of meat that were left out in the open. He therefore asked the following questions:
Where do the flies come from? Is the rotting meat transformed into the flies? From
these questions, Redi formulated the hypothesis that only flies can make flies, and
that rotting meat cannot be transformed into flies.
Redi sought to test his hypothesis by performing the following experiment. He
placed pieces of meat in three glass jars, each jar containing a piece of meat. The
first jar was left open, the second was covered with a loose netting, and the third
was completely sealed. All jars were exposed to flies in the surrounding room. Redi
predicted that if meat could not be transformed into flies, then the sealed containers should not produce either maggots or flies. Whereas if the meat can be so transformed, than the sealed jar should also develop maggots and flies.
Redi recorded the presence or absence of flies and maggots in each of the three
types of jars. As he predicted, neither flies nor maggots were found in the sealed
jars, whereas in the open jars, maggots and flies were abundant. In the jars covered
with netting, maggots were found within the netting itself, but not on the meat
inside the jar. Redi concluded that meat could not transform into flies, only flies
could produce flies, the theory of spontaneous generation could not be supported
and was incorrect.
Unfortunately, Redi's experiment did not convince everyone. Some argued that
while spontaneous generation might not apply to larger organisms like maggots
and flies, it might still be applicable to smaller microbes. The question was finally
answered definitively in the late 1800s by Louis Pasteur, in his now classic experiment.
Pasteur's Experiment
Pasteur's hypothesis was that if cells could arise from nonliving substances, then
they should appear spontaneously in sterile broth.
To test his hypothesis, he created two treatment groups: a broth that was exposed
to a source of microbial cells, and a broth that was not. For his control treatment,
Pasteur used a straight-necked flask that allowed particles in the air to fall into the
broth stored in the flask. For his experimental treatment, Pasteur used a swannecked flask. The neck shape and length assured that no cells could enter the broth
from the air.
Pasteur then performed the same procedure on these two flasks. He boiled the
broth to kill any existing organisms. He then let the broth cool and allowed it to sit
for several days, after which he removed the necks of the flasks and checked the
broth for the growth of any organisms.
Pasteur found living organisms only in the control flask. Because the experimental
flask remained sterile, the hypothesis of spontaneous generation was rejected.
By changing a single variable—the shape of the flask—Pasteur was able to conclude that cells were not generated spontaneously but were actually entering the
broth from the surrounding air.
Microorganisms, carried by dust particles, fell into the straight-necked flask.
However, the swan neck trapped the particles, preventing cells from entering the
broth.
You should now be able to…
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Compare the experiments that Redi and Pasteur used to address the possibility of spontaneous generation and specify the experimental variables
being tested in their experiments.
Explain the importance of a control in an experiment.
Describe the effects of boiling on microbial growth.