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THE CELL THEORY
The cell theory states that:
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All living organisms are composed of cells.
Cells are the smallest units of life.
All cells come from pre-existing cells
Evidence for Cell Theory:
ALL LIVING ORGANISMS ARE COMPOSED OF CELLS
The cell theory was originally articulated by scientists Theodor Schwann and Matthias
Schleiden in 1839. Schleiden summarized their observations as follows:
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The cell is the unit of structure, physiology, and organization in living things.
The cell retains a dual existence as a distinct entity and a building block in the
construction of organisms.
Cells form by free-cell formation, similar to the formation of crystals
(spontaneous generation)
We now know that Schleiden’s first two statements are correct, but the latter statement
regarding the spontaneous generation of cells is not. The idea of spontaneous generation of
cells was not disproved until Louis Pasteur’s experiments in 1862.
Cells taken from an organism often survive for a time, but smaller parts of an organism do
not. Thus, cells are the smallest units of life that are capable of survival.
CELLS ARE THE SMALLEST UNITS OF LIFE
Cells taken out from an organism, given the right condition, can survive by themselves for a
while, but organelles within the cells such as the nucleus or the mitochondria cannot.
Therefore, cells seem to be the smallest units of life because they are the smallest units that
can survive on their own.
ALL CELLS COME FROM PRE-EXISTING CELLS
Louis Pasteur ran experiments in 1862 in which he tested sterilized and heated broths. The
broths were kept in two different flasks- one with a straight neck and one with a curved neck.
In the flasks with straight necks, Pasteur allowed bacteria to enter and reside within the
broth. The broth was soon full of bacteria.
However, in the flasks with curved (S-shaped) necks, despite being exposed to the open air
there was a cotton plug that blocked the bacteria off from the broth, and no bacteria grew in
the broth within the flasks with curved necks. Once the cotton plug was removed, bacteria
grew in the broth.
This disproved Schleiden and Schwann’s
previous assumption that spontaneous
generation was what produced cells.
Pasteur’s experiment showed that so long as
there were no bacteria cells available (the Sshaped neck was still kept on,) then no
bacteria cells grew. Once the S-shaped neck
was removed, bacteria cells were able to
enter the flask and the broth and therefore
bacteria cells grew. The new bacteria cells
needed the original bacteria cells in order to
form. Thus, although the first cells must
have been formed from non-cellular
material, the succeeding cells were and only
can be formed by cell division.
Works Cited
Allott, Andrew. Biology for the IB Diploma: Standard and Higher Level. Oxford: Oxford UP,
2007. Print.
"Cell Theory." University of Miami Department of Biology. University of Miami. Web. 26 Aug.
2010. <http://www.bio.miami.edu/~cmallery/150/unity/cell.text.htm>.
Simmons, Kent. "Cell Theory." Virtual Microscope Home Page. University of Winnipeg, Fall
2007. Web. 25 Aug. 2010. <http://kentsimmons.uwinnipeg.ca/cm1504/celltheory.htm>.