Magnesium + hydrochloric acid → magnesium chloride + hydrogen

Mg + 2HCl → MgCl2 + H2

In this reaction:
Magnesium (Mg) is a
Hydrogen (H2) is a
magnesium chloride (MgCl2) is a
hydrochloric acid (HCl) is a..

Magnesium and hydrochloride acid are the reactants that are going to be produced into mgcl2 and hydrogen or h2 which will be the products.

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.