Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
THE CELL THEORY The cell theory states that: ● ● ● 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: ○ ○ ○ 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>.