Your favorite plant is growing very slowly, and you would like to find some way to increase its growth rate. which of the following should you try increasing first?
a) nitrogen
b) oxygen
c) soduim
d) calcuim

Nitrogen, because nitrogen is where plants get their protein from.

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.