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The cell theory states that all living organisms have a basic unit of structure and function, which is the cell. This was a biologically significant statement because it suggested that all living things have a common denominator. Almost 200 years of research by many different scientists led to this conclusion.
The initial discovery of cells was done by Robert Hooke, an English scientist, in 1665. Hooke designed one of the first microscopes and used it to look at plant material. One specimen he examined was thinly sliced pieces of cork. By looking at the cork through the microscope, he discovered that it was made up of many small units. Hooke named these units cells and, without realizing it, he had discovered the basis of all living matter.
Hooke and other scientists observed other samples of plant material and discovered that they were also made up of cells. As more and more material was examined, scientists began to recognize a pattern. It wasn't until 1838, however, that German scientist Matthias Schleiden stated that all plant material was made up of cells. The following year, Theodor Schwann came to the same conclusion about animals. Their findings are what have become known as the cell theory.
Originally, Hooke thought that cells were hollow and that the wall was the living portion. With the advent of better microscopes, it became evident that they were not hollow. Organelles began to be discovered, starting with the larger ones like the nucleus and chloroplast. In 1849, the first description of cell division was recorded.
With this new information, Rudolf Virchow, a German physician, proposed an adjustment to the cell theory. In 1858, he suggested that the theory be changed to include that all cells come from pre-existing cells. This was new information to most scientists and had not been understood before. Schwann, for example, thought that new cells arose from particles in the fluid surrounding them.
In the following years, experiments conducted by Louis Pasteur provided the proof for Virchow’s proposal. Pasteur carried out experiments to determine how substances like milk and wine became curdled or fermented. Through carefully controlled environments, he proved that exposure to airborne particles is what caused the change. In other words, he proved that organisms did not arise spontaneously, but had to be provided by some means, in this case in the air.
Over time, and with the development of the electron microscope, the theory has continued to evolve. As more and more living material has been observed at higher and higher magnifications, much more has been learned about cells, leading to the modern cell theory. The modern theory includes the two basic components of the classic version and then adds the following:
- Organisms can be unicellular, composed of one cell, or multi-cellular, composed of many cells.
- When cells divide, the hereditary information they contain, DNA, is passed from cell to cell.
- Energy flow occurs within cells.
- All cells have basically the same composition.
- The activity of the organism is determined by the activity of the independent cells.
The theory has two components: all living things are made up of cells and all cells arise from other cells. This gives the basis for a definition for all living things. All living things are composed of cells and all are capable of reproducing themselves.