A mechanical model illustrates the principle of coupled chemical reactions. the spontaneous reaction shown in (a) could serve as an analogy for the direct oxidation of glucose to CO2 and h2O, which produces heat only. In (B) the same reaction is coupled to a second reaction; this second reaction could serve as an analogy for the synthesis of activated carrier molecules. the energy produced in (B) is in a more useful form than in (a) and can be used to drive a variety of otherwise energetically unfavorable reactions (C