1. Observation vs. Inference Variables Spontaneous Generation
Experimental Design
Observation vs. Inference
Variables
Spontaneous Generation

2. Observation vs. Inference
Observation – something we can see or can experience through our senses (taste, feel, see, hear, etc)
Quantitative
Qualitative
Inference – something we think will happen based on our observations, something we know from prior knowledge

3. Quantitative vs. Qualitative
Recorded as measurements
Qualitative
Recorded as descriptions

4. Variables Control – used for comparison
Dependent Variable – variables that change based on the independent variable
Independent Variable – variables that the experimenter changes

5. Scientific Method Purpose Research/Hypothesis Experiment Analysis
Making predictions using observations/inferences
Research/Hypothesis
Find out as much as you can then form a hypothesis “If…Then…”
Experiment
Carry out an experiment using control and variables
Analysis
Make observations and collect data
Conclusions
Review the data and then accept/reject the hypothesis
Repeat
Make sure the results are repeatable

6. Designing an Experiment
How could you decrease a person’s blood pressure?
Hypothesis:
Control:
Dependent variable:
Independent variable:

7. Abiogenesis
Before the 1600s, it was generally thought that organisms could arise from nonliving material by spontaneous generation

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Aristotle (384 –322 BC)
Proposed the theory of spontaneous generation
Also called abiogenesis
Idea that living things can arise from nonliving matter
Idea lasted almost 2000 years
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9. Examples of Spontaneous Generation
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Example #1
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
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Example #1
Conclusion: It was perfectly obvious to people back then that muddy soil gave rise to the frogs
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Example #2
Observation: In many parts of Europe, medieval farmers stored grain in barns with thatched roofs (like Shakespeare’s house). As a roof aged, it was not uncommon for it to start leaking. This could lead to spoiled or moldy grain, and of course there were lots of mice around.
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Example 2
Conclusion: It was obvious to them that the mice came from the moldy grain.
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14. Disproving Spontaneous Generation
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Francesco Redi (1668)
In 1668, Francesco Redi, an Italian physician, did an experiment with flies and wide-mouth jars containing meat
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Redi’s Experiment
Redi used open & closed flasks which contained meat.
His hypothesis was that rotten meat does not turn into flies.
He observed these flasks to see in which one(s) maggots would develop.
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Redi’s Findings
He found that if a flask was closed with a lid so adult flies could not get in, no maggots developed on the rotting meat within.
In a flask without a lid, maggots soon were seen in the meat because adult flies had laid eggs and more adult flies soon appeared.
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18. Redi
Redi showed in 1668 that rotting meat kept away from flies would not produce new flies
Maggots appeared only on meat that had been exposed to flies

19. Did Redi Use the Scientific Method?
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The Scientific Method
Observation
Hypothesis
Experiment
Accept, Reject, or Modify hypothesis
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21. Lazzaro Spallanzani’s (1765)
Boiled soups for almost an hour and sealed containers by melting the slender necks closed.
The soups remained clear.
Later, he broke the seals & the soups became cloudy with microbes.
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22. Spallanzani’s Results
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Conclusion
Critics said sealed vials did not allow enough air for organisms to survive and that prolonged heating destroyed “life force”
Therefore, spontaneous generation remained the theory of the time
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24. Spallanzani
Spallanzani showed in the 1700s that microorganisms would not grow in broth when its container was heated and then sealed
This seemed to indicate that microorganisms that cause food spoilage do not arise from spontaneous generation but, rather, are carried in the air

25. The Theory Finally Changes
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26. Pasteur
Pasteur used a variation of Spallanzani’s design to prove that microorganisms are carried in the air and do not arise by spontaneous generation

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Pasteur's Problem
Hypothesis: Microbes come from cells of organisms on dust particles in the air; not the air itself.
Pasteur put broth into several special S-shaped flasks
Each flask was boiled and placed at various locations
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28. Pasteur's Experiment - Step 1
S-shaped Flask
Filled with broth
The special shaped was intended to trap any dust particles containing bacteria
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29. Pasteur's Experiment - Step 2
Flasks boiled
Microbes Killed
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30. Pasteur's Experiment - Step 3
Flask left at various locations
Did not turn cloudy
Microbes not found
Notice the dust that collected in the neck of the flask
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31. Pasteur's Experimental Results
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32. The Theory of Biogenesis
Pasteur’s S-shaped flask kept microbes out but let air in.
Proved microbes only come from other microbes (life from life) - biogenesis
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Figure 1.3