How Scientists Work

Key Concepts How do scientists test hypothesis? Why is it important to be able to repeat investigations?

Designing an Experiment Ideas concerning the natural world have changed over the centuries. Many people thought that living things came into being from nonliving things (maggots formed from rotting meat). About 400 years ago, people began questioning this idea.

Go to Section: Designing an Experiment Section 1-2 Flowchart State the Problem Form a Hypothesis Set Up a Controlled Experiment Record Results Analyze Results Draw a Conclusion Publish Results

Ask a Question Does life just suddenly appear? Like maggots on meat How do new living things, or organisms, come into being?

Form a Hypothesis Scientists believed in SPONTANEOUS GENERATION, the idea that life could arise from nonliving matter Francesco Redi (1668) proposed a new hypothesis: flies produce maggots.

Controlled Experiment Redi set up a controlled experiment to test his hypothesis.

Go to Section: OBSERVATIONS: Flies land on meat that is left uncovered. Later, maggots appear on the meat. HYPOTHESIS: Flies produce maggots. PROCEDURE Controlled Variables: jars, type of meat, location, temperature, time Manipulated Variables: gauze covering that keeps flies away from meat Uncovered jarsCovered jars Several days pass Maggots appearNo maggots appear Responding Variable: whether maggots appear CONCLUSION: Maggots form only when flies come in contact with meat. Spontaneous generation of maggots did not occur. Section 1-2 Figure 1-8 Redi’s Experiment on Spontaneous Generation

Record/Analyze Results Later scientists were able to learn from Redi’s work because he accurately recorded his results.

Drawing a Conclusion Redi’s results supported his hypothesis that flies produce maggots and refuted the hypothesis of spontaneous generation.

Repeating Investigations A key assumption in science is that nature behaves in a consistent manner; results should always be the same. Other’s who retested Redi’s experiment were Needham, Spallanzani, and Pasteur

Needham boiled gravy to kill any organisms. After several days, the bottle swarmed with organisms. His conclusion: “animals came from the juice of the gravy”.

Go to Section: Gravy is boiled.Flask is open. Gravy is teeming with microorganisms. Gravy is boiled. Flask is sealed. Gravy is free of microorganisms. Section 1-2 Figure 1-10 Spallanzani’s Experiment

Go to Section: Broth is boiled.Broth is free of microorganisms for a year. Curved neck is removed. Broth is teeming with microorganisms. Section 1-2 Figure 1-11 Pasteur’s Experiment