Building on these discoveries and using improving microscopes, Robert Virchow put together the growing body of evidence, and formed the cell theory; the idea that all life on earth consisted of one or more cells.

It is now thought that the first cells on earth arrived over 3.1 billion years ago, and strongly resembled what are now called prokaryotic cells.

Prokaryotic cell structure is very basic, whereas eukaryotic cells have more complex internal compartments and functions.

One problem all cells face is size. Materials must move into, around and out of cells, and the surface area to volume ration limits the size to which any cell can grow.

Diffusion, osmosis, and transport are the main routes for getting small, soluble substances into and out of cells.

However, larger amounts of material, solid or liquid, can be packaged in cell membrane, endocytosis, moved around inside the cell, and then eliminated by exocytosis.