Cell is the basic unit of life in which all the fundamental functions of living organism are marked. Cell was discovered by Robert Hooke (1665). He coined the term cell (Gk. Kytos- cell, Lt. cella-hollow space) and described it in his book Micrographia. Purkinje (1839) named the jelly-like substance of the cell as protoplasm and Robert Brown (1831) discovered the nucleus. J. Huxley termed protoplasm as ‘physical basis of life’.
M.J. Schleiden (1838) working on plant cell and T. Schwann (1839) working on animal cell established the cell theory which states that all organisms are made upon of cells and cell is the unit of structure and function of all living organisms. Rudolf Virchow (1855) postulated ‘Omnis cellula those cellula’ which means all cells arise from pre-existing cells. However, there are some exceptions to cell theory-viruses lack protoplasm, bacteria and cyanobacteria (blue-green algae- BGA) lack well organized nucleus, etc.
Cell is self-contained unit. It lives independently in unicellular organisms but shows divisions of labour in multicellular organisms. All cells are totipotent as they are capable of regenerating a whole plant.
On the basis of cellular organization, organisms are classified as prokaryotes (bacteria, BGA, mycoplasma) and eukaryotes (higher organisms). Prokaryotes cells differ from eukaryotic cells by –nucleus not organized (nuclear envelope and nucleous absent), histone protein not associated with DNA to form chromatin, membrance-bound organelles (plastids, mitochondria, ELECTRONS, peroxisome, lysosome, etc.) and true vacuole are absent, ribosome is of 70S type. Eukaryotic ribosome is 80s type.
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Higher plants and animals are Eukaryotes but plantcells differ from animal cells in having a rigid cell wall, distinct vacuoles, plastids for photosynthesis and couloration and lack of centrosome. Animal cells possess outward projections (microvill, desmosome, etc.) which are not present in plantcells.