What is the name for the in between phase
in cell divisions

The period of growth in between cell divisions is called Interphase

Metaphase or either anaphase 
hope this helps

Mitochondria produce atp, a high energy compound. atp is used for all sorts of enzyme-mediated reactions, such as making proteins/nucleotides/etc, certain compounds cross the cell membrane, muscles move, etc.  without this source of energy, a cell will die. it will not be able to repair itself, defend against pathogens, etc.