How Scientists Work The Scientific Method and Important Experiments in Biology

Spontaneous Generation Years ago scientists believed many living things appeared out of nowhere Jean Baptist van Helmont in the mid-1600s. placed a moist cloth and wheat grain inside a closed, wooden box. To his astonishment, he found a mouse nibbling at the box within a few days. Why do you think reasonable individuals once accepted these ideas?

Asking a Question People wondered how events happened (such as mice appearing from grain) They were identifying a problem! The Question –How do new living things or organisms come into being?

Forming a Hypothesis For centuries people accepted that things simply arose from non-living things This idea or “educated guess” was given the name Spontaneous generation Today we consider this a hypothesis Many hypotheses were made and tested to disprove the original hypothesis of spontaneous generation

Francesco Redi formed the hypothesis that flies produced maggots based on his experiences. He tested this hypothesis by setting up a controlled experiment! –Only one variable is changed at a time –The variable that is deliberately changed is called the independent or manipulated variable (“I Change”) –The variable that is observed to change due to manipulated variable is called the responding or dependent variable. (“What is Measured”)

Redi’s Experiment What was the Manipulated (Independent) Variable? What was the Responding (Dependent) Variable? Maggots AppearNO Maggots Appear

Recording and Analyzing Results Scientists usually keep records of their observations and data –In this class we will keep lab reports!! This is done by describing and sometimes drawing pictures of observations Today much of recording is done on computers Today scientists publish their results in scientific journals such as the one pictured above.

Repeating Investigations NOTHING IN SCIENCE IS FACT! Results are tested, and tested, and tested…to ensure that they are REPEATABLE. Scientists are expected to test each other; this ensures that the best science is being done Communication is key to good science A few scientists tested Redi’s findings…

Needham’s Test of Redi’s Findings Hypothesized that Spontaneous Generation could occur under the right conditions

Spallanzani’s Test of Redi and Needham’s Results Concluded that Needham’s test was flawed (he had not heated the broth enough) and improved upon his test Hypothesis was supported that only new organisms are produced from existing organisms

Pasteur’s Test Many scientists still argued that their were flaws in Spallanzani’s experiment (the animalcules were deprived of air). Pasteur designed a new flask and tested spontaneous generation He supported his hypothesis that all living things come from other living things. His findings helped to convince others that spontaneous generation was not correct.

Impact of Pasteur’s Work During his life Pasteur made many discoveries related to microorganisms He saved the French wine industry and the silk industry He began to uncover the nature of infectious diseases (they were the result of microorganism) He is considered one of biology’s most remarkable problem solvers

When Experiments Are Not Possible We cannot always do experiments to test a hypothesis –Ex. field studies of animal behavior (how can you study interactions in the wild in a laboratory?) –Ethical considerations What can we do when experimentation is not possible?

How a Theory Develops After much testing with supporting results a hypothesis may develop into a theory Even spontaneous generation could have been considered a theory (there was a great deal of supporting evidence) The hypothesis that new life comes from existing life is now considered the theory of biogenesis – meaning generating from life.

Theory In science the word theory applies to a well tested explanation that unifies a broad range of observations. They allow scientists to make accurate predictions about new situations It is still important to note that NO theory is considered absolute truth and as new evidence becomes available theories may be modified or thrown out completely!

Questions Answer the following questions in complete sentences: 1.Why is Redi’s experiment on spontaneous generation considered a controlled experiment? 2.How do scientists today usually communicate their results and conclusions? 3.How did the design of Pasteur’s flask help him successfully refute the hypothesis of spontaneous generation? 4.Evaluate the impact of Pasteur’s investigations on scientists’ ideas and people’s lives.