How Scientists Work and the Development of the Cell Theory

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How Scientists Work and the Development of the
Cell Theory

• Biology 112
• To follow Chapter 1, Section 2 pages 8-14
• To follow Chapter 7, Section 1 pages 169-173

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Abiogenesis

• The belief that living things could spontaneously
  come from non-living things.
• This is also called
• Spontaneous Generation.

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• Aristotle believed that frogs came from the mud.

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• Jean van Helmont believed that mice were created
  by grains of wheat and a dirty shirt because when
  the two were placed together, he found mice!

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Wheat Shirt ..
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Mice..?
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Redi or not, here I come

• Did not believe in abiogenesis.
• Observed meat rotting in jars and flies
  appeared!?
• When he covered the jars, no flies appeared!
• Conclusion Meat does NOT create fliesother
  flies create flies.
• Was criticizedothers believed that by covering
  the jars, the Active Ingredient was missing.
• Performed the experiment and covered the jars
  with mesh and got the same result.
• Arrived at the same conclusion.

Francesco Redi- 1668
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Redi did not rely on observation alone.

• He hypothesized that flies produce maggots which
  then develop into flies.
• He set up an experiment, and tested only one
  variable, the mesh covering.
• He manipulated the mesh covering on the jars so
  that he could be sure to control the presence of
  the flies on the meat.
• When flies were not in contact with the meat, no
  maggots appeared from the meat.
• He concluded that spontaneous generations was
  impossible.

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The Microscope

• Its invention revealed the presence of
  microorganisms!
• A. van Leeuwenhoek called them animalcules in
  1674.

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Needhams Test of Redis Findings 

• John Needham (mid 1700s) used an experiment
  involving animalcules to attack Redis work.
• Needham claimed spontaneous generation could
  occur under the right conditions.
• To prove this, he sealed gravy and heated it to
  kill any living things in the gravy.
•      

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Needham (continued)

• After several days he found the contents of the
  bottle swarming with activity.
• He inferred that the little animals could only
  come from the juice of the gravy.
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The basics of Needhams work
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Spallanzanis Test of Redis Findings 

• Lazzaro Spallanzani read about Redis and
  Needhams work and thought Needham had not heated
  his samples enough. He decided to improve upon
  Needhams experiment.
• He boiled 2 containers of gravy, assuming the
  boiling would kill any living thing present. He
  sealed one jar immediately and left one jar open.
  
• After a few days the gravy in the open jar had
  microorganisms and the closed one did not.
•   

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• After a few days the gravy in the open jar had
  microorganisms and the closed one did not.
•   

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Spallanzanis Test of Redis Findings 

• He concluded that nonliving gravy did not produce
  living things because the microorganisms in the
  unsealed jar were offspring from the
  microorganisms that entered the jar through the
  air.
• This experiment and Redis work supported the
  hypothesis that new organisms are produced only
  by existing organisms.

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Pasteurs Test of Spontaneous Generation(1800s) 

• Some scientists still continued to support
  spontaneous generation. Some argued that the air
  was the factor for generating life because it
  contained the life force needed to produce new
  life.
• They pointed out that Spallanzanis experiment
  was not a fair test because air had been excluded
  from the sealed jar.

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Pasteur (continued)

• Louis Pasteur, a French scientist, found a way to
  settle the argument in 1864.
• He designed a flask that had a long curved neck.
  The flask remained open to air, but
  microorganisms from the air could not make their
  way through the neck into the flask.
• He showed that as long as the broth was protected
  from microorganisms (not air) it would remain
  free of living things.

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• He broke the neck of the flask after about a year
  of experimenting and the broth quickly became
  filled with microorganisms.
• His work convinced other scientists that the
  hypothesis of spontaneous generation was
  incorrect.
• Pasteur showed that all living things come from
  other living things.
• This represented a major shift in the way
  scientists viewed living things.
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Theories

• are explanations to ideas. They have been
  well-tested!

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The development of the cell theory

• Robert Hooke was the first to describe cells
  after looking at cork under a microscope.
• Meanwhile van Leeuwenhoek was observing living
  things in pond water.
• About 200 years pass .
• Schleiden (a German botanist) was observing cells
  in plants, while Schwann (a German biologist) was
  observing cells in animals.
• ConclusionAll plants and animals are made of
  cells.
• Virchow observed and concluded that new cells
  came from other cells.

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The Cell Theory was developed based on the work
of these and other scientists

• The cell theory states that
• the cell is the basic unit of life
• all living organisms are made of one or many
  cells.
• all cells come from pre-existing cells.

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Cells can be Prokaryotic or Eukaryotic

• Prokaryotic cells
• are smaller and simpler
• do not have nuclear membrane.
• their genetic information is not contained in a
  nucleus, rather it is spread throughout the
  cytoplasm.
• Bacterial cells are an example of prokaryotic
  cells.
• Eurkaryotic cells
• are larger and more complex
• have a nucleus surrounded by a nuclear membrane.
• are found in higher life forms such as plant
  cells and animal cells.