Embryological evidence suggests that the echinoderms are closely related to

Echinoderms, like chordates, are deuterostomes and are therefore thought to be the most closely related of the major phyla to the chordates

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.