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Biology
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1-2 How Scientists Work
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3. Designing an Experiment
How do scientists test hypotheses?
Whenever possible, a hypothesis should be tested by an experiment in which only one variable is changed at a time. All other variables should be kept unchanged, or controlled.
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4. Designing an Experiment
The process of testing a hypothesis includes:
Asking a question
Forming a hypothesis
Setting up a controlled experiment
Recording and analyzing results
Drawing a conclusion
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5. Designing an Experiment
Asking a Question
Many years ago, people wanted to know how living things came into existence. They asked:
How do organisms come into being?
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6. Designing an Experiment
Forming a Hypothesis
One early hypothesis was spontaneous generation, or the idea that life could come from nonliving matter. For example, most people thought that maggots spontaneously appeared on meat.
In 1668, Redi proposed a different hypothesis: that maggots came from eggs that flies laid on meat.
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7. Designing an Experiment
Setting Up a Controlled Experiment
The variable that is deliberately changed is called the manipulated variable.
The variable that is observed and that changes in response to the manipulated variable is called the responding variable.
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8. Designing an Experiment
Redi’s Experiment
Uncovered jars
Covered jars
Controlled Variables:
jars, type of meat,
location, temperature,
time
In a controlled experiment, only one variable is tested at a time. Redi designed an experiment to determine what caused the sudden appearance of maggots. In his experiment, the manipulated variable was the presence or absence of the gauze covering. The results of this experiment helped disprove the hypothesis of spontaneous generation.
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9. Designing an Experiment
Redi’s Experiment
Manipulated Variable:
Gauze covering that keeps flies away from meat
Several days pass.
Responding Variable:
whether maggots appear
Maggots appear.
No maggots appear.
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10. Designing an Experiment
Redi’s Experiment
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11. Designing an Experiment
Recording and Analyzing Results
Scientists keep written records of their observations, or data.
Sometimes drawings are used to record certain kinds of observations.
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12. Designing an Experiment
Today, researchers use computers to record their work.
Online storage makes it easier for researchers to review the data.
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13. Designing an Experiment
Drawing a Conclusion
Scientists use the data from an experiment to evaluate a hypothesis and draw a valid conclusion.
Redi’s results supported the hypothesis that maggots were produced by flies, not spontaneous generation.
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14. Repeating Investigations
Scientists repeat experiments to be sure that the results match those already obtained.
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15. Repeating Investigations
Needham's Test of Redi's Findings
Needham challenged Redi’s results by claiming that spontaneous generation could occur under the right conditions.
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16. Repeating Investigations
Needham’s Test of Redi’s Findings
Needham sealed a bottle of gravy and heated it.
After several days, the gravy was swarming with microorganisms.
Needham concluded that these organisms came from the gravy by spontaneous generation.
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17. Repeating Investigations
Spallanzani's Test of Redi's Findings 
Gravy is boiled.
Gravy is boiled.
Spallanzani’s experiment showed that microorganisms will not grow in boiled gravy that has been sealed but will grow in boiled gravy that is left open to the air. Interpreting Graphics What variable was controlled in this experiment?
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18. Repeating Investigations
Spallanzani's Test of Redi's Findings 
Flask is open.
Flask is sealed.
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19. Repeating Investigations
Spallanzani's Test of Redi's Findings 
Gravy is teeming with microorganisms.
Gravy is free of microorganisms.
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20. Repeating Investigations
Pasteur's Test of Spontaneous Generation
Louis Pasteur conclusively disproved the hypothesis of spontaneous generation.
Pasteur showed that all living things come from other living things.
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21. Repeating Investigations
Pasteur’s Experiment
Broth is free of microorganisms for a year.
Curved neck is removed.
Broth is teeming with microorganisms.
Pasteur’s experiment showed that boiled broth would remain free of microorganisms even if air was allowed in, as long as dust and other particles were kept out.
Broth is boiled
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22. Repeating Investigations
The Impact of Pasteur’s Work
Pasteur saved the French wine industry, which was troubled by unexplained souring of wine.
He saved the silk industry, which was endangered by a silkworm disease.
He began to uncover the nature of infectious diseases, showing that they were the result of microorganisms.
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23. When Experiments Are Not Possible
It is not always possible to do an experiment to test a hypothesis. For example:
Wild animals must be observed without disturbing them.
Ethical considerations prevent some experiments.
By carefully planning alternative investigations, scientists can discover reliable patterns that add to scientific understanding.
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How a Theory Develops
How does a scientific theory develop?
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How a Theory Develops
How a Theory Develops
As evidence from numerous investigations builds up, a hypothesis may become so well supported that scientists consider it a theory.
In science, the word theory applies to a well-tested explanation that unifies a broad range of observations.
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How a Theory Develops
No theory is considered absolute truth.
As new evidence is uncovered, a theory may be revised or replaced by a more useful explanation.
In a controlled experiment, only one variable is tested at a time. Redi designed an experiment to determine what caused the sudden appearance of maggots. In his experiment, the manipulated variable was the presence or absence of the gauze covering. The results of this experiment helped disprove the hypothesis of spontaneous generation.
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1–2
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1–2
In an experiment, the variable that is deliberately changed is called the
control.
manipulated variable.
responding variable.
constant control.
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1–2
The mistaken belief that living organisms can arise from nonliving matter is called
biogenesis.
Pasteur's theory.
spontaneous generation.
Spallanzani’s hypothesis.
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1–2
Which of the following was the manipulated variable in Redi’s experiment?
the kind of meat used
the temperature the jars were kept at
the gauze covering on some jars
the kind of fly that visited the jars
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1–2
A well-tested explanation that unifies a broad range of observations is a
hypothesis.
variable.
control.
theory.
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1–2
A scientific explanation does not become a theory until
a majority of scientists agree with it.
it has been supported by evidence from numerous investigations and observations.
it is first proposed as an explanation.
it is published in a textbook.
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