Question

why might the breakdown of adp be less useful than atp when coupled to a nonspontaneous reaction?
breakdown of atp is spontaneous whereas breakdown of adp is nonspontaneous.
atp is more abundant in the cell than adp.
atp has three negative charges while adp only has two, so adp has less potential energy.
atp can be broken down in adp but adp cannot be broken down further.

Explanation:

ATP (adenosine triphosphate) has three phosphate groups. The negative charges on these phosphate groups repel each other. Breaking the bond of the out - most phosphate group releases a significant amount of energy. ADP (adenosine diphosphate) has only two phosphate groups. The repulsion between the negative charges is less compared to ATP. So, ADP has less potential energy stored in its phosphate bonds. When coupled to a non - spontaneous reaction (which requires energy input), the energy released from ATP breakdown is more useful as it can provide the necessary energy to drive the non - spontaneous reaction forward.

Answer:

ATP has three negative charges while ADP only has two, so ADP has less potential energy.