The processes of diffusion and active transport are both used to * break down molecules to release energy move molecules into or out of cells of the body bring molecules into cells when they are more concentrated outside of the cell move molecules against a concentration gradient, using ATP molecules

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.