According to cell theory all cells have a nucleus all cells are differentiated all cells reproduce by mitosis all living organisms are composed of cells

All living organisms are composed of cells

Explanation:

Before the invention of advanced and complex microscopes used today, scientists like Anton Van Leeuwenhook observed some living organisms e.g Protista using his invented lens (now ancient) but later in 1665, Robert Hooke, was able to coin the term "cell" for the box-like structures he observed when viewing corked tissues through a lens.

However, in the 1800s, this view began to change thanks to the advancement in lens and the construction of microscope, which enabled scientists to see components of the cell. Microscopes allowed early scientists to view and postulate about the cells they could see. In 1839 specifically, Scientists like Mathias Schleiden and Theodor Schwann formulated a general theory of how cells work that is very simple, which was later spiced up by Rudolf Virchow.

The universally accepted historic scientific theory called Unified Cell theory states that:

1. All cells are made up of one (unicellular) or more (multicellular) cells.

2. The cell is the basic and fundamental unit of life and;

3. New cells arise from previously existing cells