To communicate between cells, humans and animals use: 1
nerve signals and respiratory gases
2
respiratory gases and hormones
3
bones and muscles
4
nerve signals and hormones

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.