Exergonic and endergonic reactions in cells are interrelated in that...

• A. They occur in separate organelles.
• B. Endergonic reactions release energy to power exergonic reactions.
• C. Energy released from exergonic reactions powers endergonic reactions.
• D. They are the same type.

Answer: (C)

Cells use energy coupling to run processes that require energy. Exergonic reactions release free energy, and endergonic reactions need energy input. The connection happens through energy carriers like ATP. When ATP is hydrolyzed, energy is released and can be transferred to drive endergonic steps, often by transferring a phosphate group to a substrate (phosphorylation) or by other energetic intermediates. This makes the overall process energetically favorable, with a negative ΔG, so the endergonic reactions can proceed.

A common example is the synthesis of macromolecules, which requires energy input; the energy released from ATP hydrolysis powers these synthetic steps and the associated activation steps. The other ideas—that these reactions occur in separate organelles, or that endergonic reactions release energy, or that they are the same type—don’t align with how cellular energy is actually managed, since energy flow is about releasing energy to power other steps and can occur in the same or different locations depending on the context.