The work of eduard buchner and the discovery of enzymes laid the foundations for the field of

The appropriate response is biochemistry. Eduard Buchner showed that aging can be achieved utilizing dead chemicals as opposed to living yeast. He built up that biochemical responses are conceivable outside living cells. He was awarded a Nobel Prize in the year 1907 for his discovery on fermentation. 

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.