In the year (1839) Schleiden and Schwann have jointly proposed the 'Cell Theory' It states that all living organisms are made up of cells and cells are the structural and functional units of all organisms.
If we study the step by step development of cell theory we will understand how scientific methodology operates. It includes the following steps 1.observation 2.Hypothesis 3.Formulation of theory 4.modification of theory (if it warrants). Observations were made by Schleiden (1804 - 1881) a German botanist. He examined a large variety of plants and found that all of them were composed of cells. In 1838 he concluded that cells are the ultimate structural units of all plant tissues.
Schwann, a German Zoologist studied many types of animals and found that animal cells lack a cell wall and they are covered by a membrane. He also stated that animal cells and plant cells were basically identical but for the cell wall. He observed that both contain nucleus and a clear substance around it. He defined the cell as a membrane bound nucleus containing structure. He proposed a hypothesis that the bodies of animals and plants are composed of cells and their products.
Schleiden and Schwann both together discussed Schwann's hypothesis and they formulated cell theory. The important aspects of cell theory are:
All living organisms are made up of minute units, the cells which are the smallest entities that can be called living.
Each cell is made up of protoplasm with a nucleus and bounded by plasma membrane with or without a cell wall.
All cells are basically alike in their structure and metabolic activities.
Function of an organism is the sum total of activities and interaction of its constituent cells.
Viruses are biologists' puzzle. They are an exception to cell theory. They lack protoplasm, the essential part of the cell.
Bacteria and cyanobacteria (Blue Green algae) lack well organized nucleus.
Some of the protozons are acellular.
The coenocytic hyphae of some fungi eg. Rhizopus have undivided mass of protoplasm, in which many nuclei remain scattered.
Red Blood Corpuscles (RBC) and mature sieve tubes are without nuclei.
A cell may grow, secrete, divide or die while its adjacent cells may lie in a different physiological state. Many of the subsequent findings about the cell like this had necessitated modification in cell theory. The modified form of cell theory has been given the higher status as cell principle or cell Doctrine.
The important features of cell doctrine are:
All organisms are made up of cells.
New cells are produced from the pre-existing cells.
Cell is a structural and functional unit of all living organisms.
A cell contains hereditary information which is passed on from cell to cell during cell division.
All the cells are basically the same in chemical composition and metabolic activities.
The structure and functionof the cell are controlled by DNA.
Sometimes the dead cells may remain functional as tracheids and vessels in plants and horny cells in animals.