The energy for an endergonic reaction comes from a(n) _____ reaction.

• A. isothermal
• B. exergonic
• C. spontaneous
• D. endothermic

Answer: (B)

Verified step by step guidance

1. Understand the definitions of the terms involved: An endergonic reaction requires an input of energy to proceed, meaning it is not spontaneous and has a positive Gibbs free energy change (\$\Delta G > 0\$).
2. Recall that an exergonic reaction releases energy and is spontaneous, with a negative Gibbs free energy change (\$\Delta G < 0\$).
3. Recognize that in biological and chemical systems, the energy needed for an endergonic reaction is often supplied by coupling it to an exergonic reaction, which provides the necessary free energy.
4. Note that 'isothermal' refers to constant temperature conditions and does not directly describe energy flow in reactions, while 'endothermic' refers to heat absorption but does not necessarily indicate spontaneity or free energy changes.
5. Conclude that the energy for an endergonic reaction comes from an exergonic reaction, as this is the process that provides the free energy required to drive the non-spontaneous reaction forward.