The Scientific Method, The Cell Theory
January 10, 2001
Readings: BioInquiry Ch 1: pps 10-15, and Ch 4: pps 85 - 87, 110-115
"If we knew what we were doing, it wouldn't
be called research, would it?" --Albert
I. How is Biology studied?
Biology is an ongoing record of discoveries that arise from people asking questions about the natural world. Is is based on the idea that natural phenomena have natural causes that can be discovered by testing.
Questions, and the search for their answers, form the basis of all sciences, and science is limited to questions that can be tested.
Explanations are sought using an approach called the Scientific Method. This approach requires evidence to answer questions. Note: The scientific method is not a rigid set of rules that must all be done; but makes up the 'common threads' of most successful experiments.
1. Observation: Observe some aspect of nature.
2. Questioning: "Why do...?" "What happens if I...?" "How does...?"
3. Develop a hypothesis (a possible explanation that may answer the question)
Make a prediction of what the outcome would be if the hypothesis were true
This is called deductive, or "if-then", reasoning
4. Test the prediction by experiments, observations, or models.
Usually, a literature search is done before doing the test to avoid "reinventing the wheel".
If the tests do not confirm the prediction, determine whether the hypothesis may need to be modified
If the tests do confirm the prediction, repeat the tests for consistency
5. Explain the results and Report the conclusions via papers in scientific journals (peer-reviewed.)
II. Experimentation: The Key to the Scientific Method
A key ingredient of the scientific process: the controlled experiment.
Control group: the group in which all factors are held constant
Controls are essential for comparison with the experimental group.
Setting up the right controls is crucial for good experimental design.
Experimental group: the group in which one factor or treatment is varied
Experiment: Why are we sleepy in N100?
Worksheet: Scientific Method
The "Science Cycle":
III. About the words "hypothesis", "theory", and "law":
1. For a hypothesis to be valid, it must be able to be tested.
A hypothesis is always a tentative explanation - and must be falsifiable.
A hypothesis can be disproved, but never proved with absolute certainty!
2. A theory is a broad explanation that synthesizes many different once-unrelated facts observations, facts, and findings to explain natural processes or phenomena. Theories are very well-supported by available evidence and very widely accepted by the scientific community (ie: the Cell Theory, Evolutionary Theory)
The scientific community accepts a theory that stands up to continual testing and best explains the available evidence, and discards a theory that is inconsistent with current information.
Theories provide a framework to explain the known information of the time, but are are subject to constant evaluation and updating. Because biology and science is an "ongoing record", new information is always being reported - and theories can, and do change over time as more knowledge becomes available. There is always the possibility that new evidence will conflict with a current theory.
This is a strength of science, not a weakness! Old ideas must sometimes be given up in light of new evidence that better explains the theory or hypothesis.
Some examples of theories that have been rejected because they are now better explained by current knowledge:
Some examples of theories that were initially rejected because they fell outside of the accepted knowledge of the time, but are well-accepted today due to increased knowledge and data include:
- The sun-centered solar system
Warm-bloodedness in dinosaurs
The germ-theory of disease
Heliobacter pilorii infections as the causitive agent of many stomach ulcers
Transposons or "jumping genes"
The Endosymbiotic Theory
Note: The popular usage of the word theory differs fron the scientific use of the word theory! When people say "I have a theory about why the Supreme Court refused to allow further recounts in Florida..." they generally do not mean that they have gathered a large body of facts and findings about this matter. Really, they may have an idea, opinion, or a hypothesis about this matter.
3. Laws (ie: the Law of Gravity) are principles or generalizations about phenomena. A law is not an explanation. A theory would attempt to explain why the Law of Gravity operates as it does, but currently, there is no well-accepted 'Theory of Gravity'! (Some possible theories include String Theory, Einstein's Theory of General Relativity, etc)
Note: Hypothesis do not 'become' theories and Theories do not 'become' laws (like, for example, a bill becomes a law in congress.) Each one is a different feature of the scientific method.
IV. The Cell Theory:
The discovery of the cell was made possible
by the invention of the microscope
Robert Hooke, 1662: cork appeared as a "a great many little boxes" (or "celles", meaning 'small rooms')
Anton van Leewenhook, 1673: Observations of pond water: "Little eels, or worms, lying all huddled up together and wriggily, and the whole water seemed to be alive with these animalcules and cells fill'd with juices."
By the early 1800s, cells were found to be the basic building block of all life studied at that time.
Matthias Schleiden and Theodor Schwann, in 1839, brought 200 years of scattered observations together into a simple, testable Cell Theory:
1. The cell is the fundamental unit of life.
2. All organisms are composed of one or more cells.
At this time, there was a widely held Theory of Spontaneous
Generation - that meat spontaneously generated maggots, broth or milk
spontaneously generated bacteria, etc.
Louis Pasteur's work provided evidence contrary to this theory by covering meat to prevent flies from laying their eggs on the surface, sealing flasks from the air to prevent contamination by microbes, and concluded that "There is no circumstance known in which it can be affirmed that microscopic beings come into the world without...parents similar to themselves".
This, as well as findings from other scientists of that time studying animal and plant reproduction and embryo development, reproduction of bacteria and algae, provided evidence for the third part of the cell theory:
3. All cells come from preexisting cells.
Each one of these three tenets of the Cell Theory
Each one has been repeatedly tested.
Each one is potentially falsifiable.
|Objectives, 01 /10/ 01|
1. Know the steps of the scientific method
2. Explain the need for a control group whenever experiments are performed
3. Explain the use of the word theory in the scientific process
4. Explain the 3 tenets of the Cell Theory, and the names of the scientists involved.
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