An example of von baer's principles is the pharyngeal pouches of mammals that are indistinguishable from aquatic gill pouches in early development. in mammals the pouches do not develop in to gills, but form the: (list three of the five structures)

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.