Cell theory is the scientific theory of the unit of living world. Now a days it is universally accepted that all living organisms are made up of single unit called cell. So the cell is structural and functional unit of life.
The work of German botanist, Matthias Jakob Schleiden on plant cells and the German zoologist, Theodor Schwann on animal cells, both published in 1839, led to a clearer definition of cells and their function. They formed the basis of the cell theory, which states that all living matter, starting from the simplest unicellular organisms to very complex higher organisms (plants and animals) is composed of cells and each cell can act independently but also functions as an integral part of the complete organism.
Both Matthias Jakob Schleiden and Theodor Schwann were not clear about how the new cells are formed. The observations of Rudolf Virchow in 1855 showed that all cells are derived from the other living cells. This supplemented the old cell theory with a concept. The four main features of the modern cell theory may be summarized as follows;
- All living organisms are made of cells and their products.
- Cells are the fundamental units of structure and function in living organisms.
- The cells are units of heredity because of the presence of hereditary material in the cell.
- New cells arise or developed from the division of pre-existing cells only.
The modern version of Cell Theory State that:
- Cells are fundamental units of structure and function in all living organisms (i. e. body of all living organisms is an assemblage of cells).
- Cells are physiological units of living organisms i. e. the metabolic activities of living organisms are performed within the cells.
- Cells are hereditary units (these maintain continuity through the hereditary material).
- New cells originate from the pre-existing cells only.
- The Cell is the smallest unit of life. All activities of living organisms are the outcome of the activities of its constituent cells.
Significance of Cell Theory:
Modern concept of cell theory emphasizes the structural and functional relationship among the diverse living forms from bacteria to man.
All cells irrespective of their function and position have a nucleus embedded in the cytoplasm and are bounded by cell membrane (unity in structural plan) and the same metabolic processes occur in all the cells primitive or specialized (unity of function). This implies that all the living things have originated from the same primitive ancestral type that originated about two billion years ago.
Exceptions to Cell Theory (The Viruses):
The cell theory is one of the fundamental generalizations of biology and has universal application except for viruses, and blue green algae in which the nuclear material does not form a distinct nucleus and especially the viruses which are not complete cells.
Viruses are incomplete cell. Cellular organization is absent. They are made up of Capsid and genetic materials. They have a central core of nucleic acid (DNA or RNA) enclosed in a protein covering). They do not have their own enzyme system and depend on the enzyme system of the host cell for their metabolic activities. For this reason, viruses can multiply only inside the living cells and are called obligatory parasites. During infection only nucleic acid of virus enters the host cell and takes the command of synthetic machinery of host cell. The host cell now Starts synthesizing viral nucleic acid and viral proteins which form new viral molecules.
Viruses are very small. Some viruses are even smaller than many macromolecules of biological origin. But some viruses are even larger than some known bacteria. Based on the host in which viruses multiply, these are called as :
- Bacteriophages : Viruses that attack bacteria.
- Cyanophages : Viruses of blue green algae.
- Phytophages : Viruses that infect plant cells.
- Animal viruses : Viruses that infect animal cells.
Viruses can be crystallized just like other inorganic and organic compounds. Therefore, the living nature of viruses is debatable. The living and nonliving characters of viruses are summarized below :
Focus on Living and Non-living characters of Viruses :
Non-living characters :
- Property of crystallization like other inorganic and organic compounds.
- Absence of enzyme system.
- Lack of response to external stimuli.
- Absence of growth.
Living characters :
- Capacity to multiply (through inside some living system).
- Property of undergoing mutations.
- Property of recombination and inheritability.
- Fertile organism reproduce progeny similar to itself.
- Living things has self consciousness.
- All living organisms made up of chemicals.
If you are interested in biology also read this post – Describe the mechanism of Breathing, how breathing reflex is regulated in the mammal