An advantage of the method of organism classification is that the system is hierarchical and implies relatedness in organisms grouped together in a genus.

The classification system set by Carl Linnaeus is hierarchical. The utmost level is the level of domain, right below it is the level of kingdoms, followed by the phylum, class, and genus. The lower the level of hierarchy it is the organisms that belong there are more closely related.

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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.