Cell Theory

The three
basic tenets of the cell theory are:

1.) All living things are made of
cells

2.) Cells are the smallest structural and functional unit of living
organisms

3.) All cells come from preexisting cells.

The
first is self-explanatory, as all living things - from microscopic organisms to the largest
whale - are made of cells.  These cells can vary widely, but all living things are composed of
cells.

Secondly, living organisms cannot be broken down any farther than the
cell level. Like atoms and matter (atoms are the smallest units of matter), cells and living
organisms are related: living organisms, at their most basic level, are made of cells.

Finally, no cell comes about in a vacuum.  All cells on the planet came from other
cells. Cells divide by a number of mechanisms, such as mitosis (non-sexual cell division) and
meiosis (sexual cell division), but every cell in existence came from another
cell.