Question

what is cell theory and who proposed it.

Answer 1

Mathias Schleiden, a German botanist and Theodore Schwann, a British zoologist together formulated the cell theory in 1839. Although there was another scientist named Rudolf Virchow who modified the hypothesis of the cell theory given by Schleiden and Schwann. So altogether the cell theory described as:
i) The cell is the most basic unit of life.
ii) All living organisms are composed of cells and products of cells.
iii) All cells arise from pre-existing cells.