1. Honors Biology Chapter 1 Section 2 How Scientists Work Essential Question: How would you explain the scientific method?

2. Designing an Experiment: back in time

3. Spontaneous Generation  Life can spring from nonliving things  Based on many observations  People were convinced it worked  No one tested this hypothesis

4. Controlled Experiments  An experiment in which only one variable is changed at a time; all other variables are kept “controlled” or unchanged, or constant.

5. Variables  manipulated variable: variable that is changed by experimenter, aka: independent variable  responding variable: variable that is observed or measured in the experiment, aka: dependent variable

6. Redi (1678) 1 st to test spontaneous generation  New Hypothesis: flies produce maggots  Prediction: keep flies away from meat & no maggots will appear

7. Redi’sExperiment  Working in pairs identify the following:  Manipulated Variable  Responding Variable  Was this a controlled experiment?

8. Redi’s Experiment

9.  Results:  + maggots on control meat  - maggots on jar covered with gauze  Conclusion:  Redi’s hypothesis supported…………… So: spontaneous generation refuted

10. Repeating the Experiment  When 1 variable manipulated in experiment then repeating the experiment should always give you the same results  Scientists repeat their experiments to verify results then communicate with others  expectedthat other scientists will repeat each other’s work: it adds confidence to conclusions

11. Needham Tests Redi’s Work  Mid 1700’s English scientist John Needham tried to prove spontaneous generation could occur under certain conditions

12. Spallazani Tests Redi’s Work  Italian scientist who read Redi’s & Needham papers  hypothesized that Needham had not heated samples sufficiently

13. Spallazani’s Conclusion  organisms came from the air not from the gravy….. There is no spontaneous generation

14. Pasteur’s Final test of Spontaneous Generation  mid-1800’s arguments still going on for/against spontaneous generation  Louis Pasteur designed experiment that finally put the arguments to rest

15. Louis Pasteur  1 year later Pasteur broke neck of flask and  microrganisms appeared in the broth  Confirmed “all living things come from other living things”

16. When experiments are not possible  Not all hypotheses can be easily tested  Examples:  ?  Studies:  Not experiments in true sense  Observations  Use large #s or long periods of time to minimize risk of misleading results

17. Scientific Theories  Theory: well-tested explanation that unifies a broad range of observations  Enable scientists to make accurate predictions about new situations  Cannot be absolutely proven  Can change if new evidence disproves the theory