Which is the largest organizational level found in one coral reef?

Biome, though some might say ecosystem instead. Biomes are very large ecosystems that cover broad areas.If they specify "one" reef, then they are probably looking for "ecosystem"

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.