Honors Biology Chapter 1 Section 2 How Scientists Work Essential Question: How would you explain the scientific method?
Designing an Experiment: back in time
Spontaneous Generation Life can spring from nonliving things Based on many observations People were convinced it worked No one tested this hypothesis
Controlled Experiments An experiment in which only one variable is changed at a time; all other variables are kept “controlled” or unchanged, or constant.
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
Redi (1678) 1 st to test spontaneous generation New Hypothesis: flies produce maggots Prediction: keep flies away from meat & no maggots will appear
Redi’sExperiment Working in pairs identify the following: Manipulated Variable Responding Variable Was this a controlled experiment?
Redi’s Experiment
Results: + maggots on control meat - maggots on jar covered with gauze Conclusion: Redi’s hypothesis supported…………… So: spontaneous generation refuted
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
Needham Tests Redi’s Work Mid 1700’s English scientist John Needham tried to prove spontaneous generation could occur under certain conditions
Spallazani Tests Redi’s Work Italian scientist who read Redi’s & Needham papers hypothesized that Needham had not heated samples sufficiently
Spallazani’s Conclusion organisms came from the air not from the gravy….. There is no spontaneous generation
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
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”
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
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