The primary chemical used in the synthesis of ATP (adenosine triphosphate) is adenosine diphosphate (ADP). During cellular respiration, ADP combines with inorganic phosphate (Pi) in a process called phosphorylation, often facilitated by the enzyme ATP synthase. This occurs in the mitochondria of cells, where energy derived from nutrients, particularly glucose, is harnessed to produce ATP. ATP acts as the primary energy currency of the cell, supplying the necessary energy for various biological processes such as muscle contraction, nerve impulse transmission, and biosynthesis.