Re: when human cells multiply, where do the additional atoms come from?

Cells get their food for matabolic and catabolic activities from the 
outside world. Transportation of food for these activities either by blood 
circulation or through other means as in case of plants is an 
indespensable part to keep the cells alive. 

A cell is a dynamic system. The non living materials enter in and out of 
the cell through many physical and bio-chemical processes. A cell can be 
considered as a complex bio-chemical engine.

Cells do synthesize bio-molecules required for division. These include 
proteins, duplication of DNA, etc. The basic raw materials required for 
these processes are required to be supplied from outside. The supply 
process itself is quite complex in multi-cellular organisms. Most of these 
things are controlled by cell membrane.