Cell Theory - What is Cell Theory?
The cells that Hooke observed were non-living cells, and the microscope he used was not advanced enough to allow him to see any cell contents, such as a nucleus or other organelles. The first person to study a live cell under a microscope was Antony van Leeuwenhoek. In 1674 he described what is today known as Spirogyra, an algae. He named the moving organism an "animicule," or "little animal."
Building on the work of Hooke and Van Leeuwenhoek, a German botanist Matthis Jakob Schleiden examined many plant samples under the microscope and came to recognize that all plants and parts of plants were composed of cells. Later on, while dining with his colleague Theodor Schwann, a zoologist, Schleiden discussed his research. Schwann had made similar findings in his research of animal cells, and in 1839 he published the work "Microscopic Investigations on the Accordance in the Structure and Growth of Plants and Animals." In this work, the first statements of cell theory were put forth, as follows:
1. The cell is the unit of structure, physiology, and organization in living things.
2. The cell retains a dual existence as a distinct entity and a building block in the construction of organisms.
3. Cells form by free-cell formation, similar to the formation of crystals (spontaneous generation).
Classical Cell Theory
In 1858, Rudolf Virchow researched and built upon the theories of Schleiden and Schwann. It was he who proposed the theory that all living cells must rise from pre-existing cells. While this may seem obvious to students of science today, at the time it was quite a radical idea and was in direct opposition to the third tenet of the cell theory proposed by Schleiden, as noted above.
Until this time, most scientists believed in the theory of "spontaneous generation," which proposed that non-living material could spontaneously generate into living matter. One of the examples of spontaneous generation often presented was the appearance of maggots on a piece of rotting meat. They were not there, and then they were there, with no discernible method of locomotion to the meat. Hence, the living maggots were believed to have spontaneously generated from the non-living meat.
It was the work of Louis Pasteur that provided the data necessary to disprove the theory of spontaneous generation. Pasteur performed experiments under controlled environments that showed how substances like broth and milk became curdled or spoiled due to exposure to airborne particles, not by spontaneous generation.
As a result of this new information, in 1858 Virchow proposed a revised cell theory, now known as the "classical cell theory," as follows:
1. All living organisms are made up of one or more cells.
2. Cells are the basic unit of life.
3. All cells arise from pre-existing cells.
4. The cell is the unit of structure, physiology, and organization in living things.
5. The cell retains a dual existence as a distinct entity and a building block in the construction of organisms.
Modern Cell Theory
Over the past 150 years, as a result of more research and improved scientific equipment, the generally accepted "modern cell theory" is now as follows:
1. The cell is the fundamental unit of structure and function in living organisms.
2. All cells arise from pre-existing cells by division.
3. Energy flow (metabolism and biochemistry) occurs within cells.
4. Cells contain hereditary information (DNA) which is passed from cell to cell during cell division.
5. All cells are basically the same in chemical composition in organisms of similar species.
6. All known living things are made up of one or more cells.
7. Some organisms are made up of only one cell and are known as unicellular organisms.
8. Others are multi-cellular, composed of a number of cells.
9. The activity of an organism depends on the total activity of independent cells.
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