Many times, the molecules join to form long chains with what kind of backbone?

Carbon backbone

Carbon is the most versatile element on the planet that allows numerous kinds of molecules to from from its backbone.

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.