Why do cells use ATP to drive endergonic reactions?

• A. Energy released by ATP hydrolysis makes ΔG for coupled reactions more negative
• B. ATP simply increases temperature to drive reactions
• C. ATP lowers activation energy
• D. ATP provides electrons for redox reactions

Answer: (A)

The idea tested is energy coupling: cells use the energy released by ATP hydrolysis to drive reactions that would otherwise require input of energy. When ATP is hydrolyzed, it releases a substantial amount of free energy. If that energy is used to convert a reactant into a higher-energy intermediate or to push a reaction forward, the overall free energy change (ΔG) for the combined steps becomes more negative. In other words, the endergonic reaction and ATP hydrolysis together have a negative ΔG, so the process can proceed spontaneously. A classic example is the phosphorylation of substrates like glucose, where transferring a phosphate from ATP to the substrate makes the overall reaction favorable. This is why ATP is described as providing the energy “currency” to power cellular work. The other options don’t capture this mechanism: ATP doesn’t raise temperature to drive reactions, nor does its primary role in most cases be to lower activation energy or provide electrons for redox reactions.