Here's why:

* Endergonic reactions: These reactions require energy input to occur. They have a positive free energy change (ΔG > 0) and are not spontaneous.

* Exergonic reactions: These reactions release energy. They have a negative free energy change (ΔG < 0) and are spontaneous.

Coupling is the process where an exergonic reaction is linked to an endergonic reaction. The energy released by the exergonic reaction is used to drive the endergonic reaction, making it possible. This is a fundamental principle in biological systems.

Example:

A common example is the synthesis of ATP (adenosine triphosphate), the energy currency of cells. ATP synthesis is endergonic. It's coupled to the exergonic breakdown of glucose during cellular respiration. The energy released from glucose breakdown is used to drive ATP synthesis.

In summary: The energy needed to power an endergonic reaction comes from a coupled exergonic reaction that releases energy. This coupling allows cells to carry out reactions that would otherwise not be possible.