In biology, a coupled reaction is best described as which of the following?

• A. Endergonic reactions release energy to drive exergonic reactions.
• B. Coupled reactions transfer energy between organisms.
• C. Coupled reactions occur only in metabolism.
• D. The energy released by an exergonic reaction is used to drive an endergonic reaction.

Answer: (D)

The idea being tested is that cells power energetically unfavorable processes by linking them to favorable, energy-releasing reactions. In energy coupling, the energy released by an exergonic reaction (one with a negative ΔG) is used to drive an endergonic reaction (one with a positive ΔG). This keeps the overall process spontaneous because the combined ΔG is negative. A common way this happens is through ATP: hydrolysis of ATP releases energy, which can then be used to phosphorylate a substrate or power a motor protein or active transport, making the endergonic step proceed.

So the best description is that the energy released by an exergonic reaction is used to drive an endergonic reaction. This concept is not about energy transfer between organisms, nor is it restricted only to metabolism, and it isn’t that endergonic reactions themselves release energy.