Duty is defined as intermittent operation in which the load conditions are regularly recurrent.

This is true, and is more specifically termed as periodic duty. This concept differs from other types of duty in that the load conditions are the same during each period, and the periods take place at intervals that are predictable. Other types of duty have intervals that are less certain.

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.