Carl Woese and his colleagues built a tree of life by comparing rRNA ribonucleotide sequences of a variety of living organisms. What novel findings arose from this analysis

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.