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

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

Aristotle believed that frogs came from the mud.

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!

Wheat + Shirt =…..

Mice…..?

“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 flies…other flies create flies. Was criticized…others 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

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.

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

Needham’s Test of Redi’s Findings John Needham (mid 1700s) used an experiment involving animalcules to attack Redi’s 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.          

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.      

The basics of Needham’s work

Spallanzani’s Test of Redi’s Findings Lazzaro Spallanzani read about Redi’s and Needham’s work and thought Needham had not heated his samples enough. He decided to improve upon Needham’s 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.    

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

Spallanzani’s Test of Redi’s 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 Redi’s work supported the hypothesis that new organisms are produced only by existing organisms.

Pasteur’s Test of Spontaneous Generation (1800’s) 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 Spallanzani’s experiment was not a fair test because air had been excluded from the sealed jar.

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.

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.      

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

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. Conclusion:All plants and animals are made of cells. Virchow observed and concluded that new cells came from other cells.

The cell theory states that : 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.

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.