This macromolecule is a source of short-term energy.

light dependent reactions:

release oxygen during the photolysis of wateroccur in the granaproduce atp

light independent reactions:

fix carbon dioxide occur in the stromaproduce glucose

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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.