H.J. Dutrochet (1924) a French worker gave the idea of cell theory.
The actual credit for cell theory goes to two German scientists, a Botanist M.J. Schleiden (1838) and a Zoologist T. Schwann(1839).They gave the concept �all living organisms are composed of cell�. Schleiden and Schwann both supported the theory of �spontaneous generation�. They also mentioned that �the new cell arises from nucleus by budding�. Main postulates of cell theory are :
(a) Living beings are made of cells. They may be unicellular, colonial or multicellular.
(b) Cell is a mass of protoplasm having nucleus.
(c) Cells are similar in structure and metabolisms.
(d) The functions of an organism are due to activities and interactions of cells.
# Exceptions to the cell theory : Viruses, viroids and prions are an exception to the cell theory as they are obligate parasites (sub�cellular in nature). Paramecium, Rhizopus, Vaucheria are some examples, which may or may not be exceptions to the cell theory.
# Modification of cell theory : Modification of cell theory was done by Rudolf Virchow (1885). He proposed the �law of cell lineage� which states that cell originates from pre-existing cells. i.e. (omnis cellula-e-cellula). It is also called �cell principle� or �cell doctrine�. It states : �
(a) Life exists only in cells.
(b) Membrane bound cell organelles of the protoplasm do not survive alone or outside the protoplasm.
(c) Cells never arise de novo. The new cells are like the parent cell in all respect.
(d) All cells have similar fundamental structure and metabolic reactions.
(e) Cells display homeostasis and remain alive.
(f) Functions of an organism as a whole are the sums of the activities and interactions of its constituent cell units. An organism can not show functions which is absent in its cells.
(g) Genetic information is stored in DNA and expressed within the cells.
(h) DNA controls structure and working of a cell.
# The cell as a self contained unit : Autonomy of a cell is believed due to presence of DNA and its expressibility, otherwise, cell components have different shape and function. It has two positions.
# Autonomy in unicellular organisms : Unicellular organisms lead to a totally independent life due to different shape, size and role of different organelles shows division of labour. All these display homeostasis. Unicellular organisms are more active due to large surface volume ratio.
# Autonomy in multicellular organisms : In multicellular organisms life activities are displayed by each of the cells independently. Multicellular organisms have one thing advantage over unicellular organisms is division of labour.
# Cellular totipotency : Totipotency was suggested by Haberlandt (1902). When cells have tendency or ability to divide and redivide the condition of the cell is called totipotent and this phenomenon is called totipotency.
# Steward�s experiment : Steward et.al. showed the phenomenon of cellular totipotency in carrot culture. Small fragments (phloem) of mature carrot roots were placed in liquid medium in special containers and growth factors like �coconut milk� was added. The culture developed into clumps or embryoids. When these were shifted to semisolid media, full plants were formed. The plants flowered normally and even bore the seeds.
# Surface volume ratio : Metabolically active cells are small, as small cells have higher nucleocytoplasmic ratio for better control and higher surface volume ratio for quicker exchange of materials between the cell and its outside environment. Larger cells have lower surface volume ratio as well as lower nucleocytoplasmic ratio. Surface volume ratio decreases by one half if cell size doubles.
H.J. Dutrochet (1924) a French worker gave the idea of cell theory.
The actual credit for cell theory goes to two German scientists, a Botanist M.J. Schleiden (1838) and a Zoologist T. Schwann(1839).They gave the concept �all living organisms are composed of cell�. Schleiden and Schwann both supported the theory of �spontaneous generation�. They also mentioned that �the new cell arises from nucleus by budding�. Main postulates of cell theory are :
(a) Living beings are made of cells. They may be unicellular, colonial or multicellular.
(b) Cell is a mass of protoplasm having nucleus.
(c) Cells are similar in structure and metabolisms.
(d) The functions of an organism are due to activities and interactions of cells.
# Exceptions to the cell theory : Viruses, viroids and prions are an exception to the cell theory as they are obligate parasites (sub�cellular in nature). Paramecium, Rhizopus, Vaucheria are some examples, which may or may not be exceptions to the cell theory.
# Modification of cell theory : Modification of cell theory was done by Rudolf Virchow (1885). He proposed the �law of cell lineage� which states that cell originates from pre-existing cells. i.e. (omnis cellula-e-cellula). It is also called �cell principle� or �cell doctrine�. It states : �
(a) Life exists only in cells.
(b) Membrane bound cell organelles of the protoplasm do not survive alone or outside the protoplasm.
(c) Cells never arise de novo. The new cells are like the parent cell in all respect.
(d) All cells have similar fundamental structure and metabolic reactions.
(e) Cells display homeostasis and remain alive.
(f) Functions of an organism as a whole are the sums of the activities and interactions of its constituent cell units. An organism can not show functions which is absent in its cells.
(g) Genetic information is stored in DNA and expressed within the cells.
(h) DNA controls structure and working of a cell.
# The cell as a self contained unit : Autonomy of a cell is believed due to presence of DNA and its expressibility, otherwise, cell components have different shape and function. It has two positions.
# Autonomy in unicellular organisms : Unicellular organisms lead to a totally independent life due to different shape, size and role of different organelles shows division of labour. All these display homeostasis. Unicellular organisms are more active due to large surface volume ratio.
# Autonomy in multicellular organisms : In multicellular organisms life activities are displayed by each of the cells independently. Multicellular organisms have one thing advantage over unicellular organisms is division of labour.
# Cellular totipotency : Totipotency was suggested by Haberlandt (1902). When cells have tendency or ability to divide and redivide the condition of the cell is called totipotent and this phenomenon is called totipotency.
# Steward�s experiment : Steward et.al. showed the phenomenon of cellular totipotency in carrot culture. Small fragments (phloem) of mature carrot roots were placed in liquid medium in special containers and growth factors like �coconut milk� was added. The culture developed into clumps or embryoids. When these were shifted to semisolid media, full plants were formed. The plants flowered normally and even bore the seeds.
# Surface volume ratio : Metabolically active cells are small, as small cells have higher nucleocytoplasmic ratio for better control and higher surface volume ratio for quicker exchange of materials between the cell and its outside environment. Larger cells have lower surface volume ratio as well as lower nucleocytoplasmic ratio. Surface volume ratio decreases by one half if cell size doubles.
