Which of the following defines an endergonic reaction?

• A. The energy change is positive and the reaction requires energy
• B. The activation energy is zero
• C. The reaction energy is exactly zero
• D. The reaction has a negative ΔG and is spontaneous

Answer: (D)

Endergonic reactions absorb energy; their free-energy change is positive (ΔG > 0), so the products have higher energy than the reactants. Because energy must be supplied, these steps are not spontaneous on their own and are often driven by coupling to energy-releasing processes (like ATP hydrolysis) elsewhere in the system. Activation energy is the barrier to start a reaction and isn’t what defines endergonic versus exergonic. If ΔG were zero, the system would be at equilibrium, not endergonic or exergonic. A negative ΔG indicates the reaction releases energy and can proceed spontaneously (exergonic). So the defining statement for an endergonic reaction is that the energy change is positive and energy is required. The contrasting statement about negative ΔG describes an exergonic, not endergonic, reaction.