What would indicate that a reaction is endergonic?

An endergonic reaction is characterized by a positive change in Gibbs free energy (ΔG), which indicates that the reaction requires free energy input in order to proceed. When ΔG is positive, it means that the products of the reaction have more free energy than the reactants, and thus the system is not at equilibrium and will need an external energy source (like light, heat, or a coupled exergonic reaction) to drive the reaction forward.

In contrast, a negative ΔG would indicate an exergonic reaction, which releases energy and is typically spontaneous. Therefore, the presence of a positive ΔG is the key indicator of an endergonic reaction, clearly differentiating it from those reactions that either release energy or proceed spontaneously without additional energy input.