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Online Biology Notes For Class 11 And Solutions – Unit 3 – Types Of Cell

Online Biology Notes For Class 11  And Solutions – Unit 3 – Types Of Cell

Different scientists proposed the definition of a cell in different ways like-
  • Mathias Jacob Schleiden, a German botanist, (1804-1881) and Theodor Schwann, a German zoologist, (1810-1882), together postulated for the first time that the cell is the basic unit of structure and function in all life.
  • G. Loewy and P. Siekevitz (1963) have defined a cell as a unit of biological activity delimited by a semipermeable membrane and capable of self-reproduction in a medium free of other living systems.
  • Wilson and Morrison (1966) defined the cell as an integrated and continuously changing system.
  • John Paul (1970) has defined the cell as the simplest integrated organization in living systems capable of independent survival.
All the above definitions have to define viruses as neither an organism nor a cell, though it contains a core of nucleic acid (DNA/RNA as genetic material) which is enclosed in an external mantle of protein. In a free state, viruses are quite inert. They become activated only when they infect a living host cell and in this process, only the nucleic acid core enters the host’s cell. The nucleic acid, which is the genetic substance, takes the metabolic machinery for its multiplication, ultimately host killing the host cell. Viruses are primitive and simpler units of life, which may be considered as cellular parasites that require the host system for its reproduction.
Broadly the cells are of two types; Prokaryotic cells and Eukaryotic cells. All unicellular organisms (single cell) including bacteria and archaea, which are composed of a single cell, are examples of prokaryotic cells. All multicellular organisms (many or more than one cell)protists, fungi, plants, and animals, including humans, are examples of Eukaryotic cells. Both prokaryotic cells and eukaryotic cells have cytoplasm, cell membrane, and genetic material.

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