Final answer:
An exergonic reaction releases free energy and can occur spontaneously, although it requires activation energy at the start. Endergonic reactions, on the other hand, require an input of energy to proceed.
Step-by-step explanation:
An exergonic reaction is characterized by a net release of free energy. These reactions have a negative change in Gibbs free energy (∆G < 0) and can occur spontaneously. Nonetheless, it's important to note that exergonic reactions still require a small amount of energy input at the start, known as activation energy, before they can proceed with their energy-releasing steps. In contrast, endergonic reactions require energy to proceed and are characterized by a positive change in Gibbs free energy (∆G > 0). The distinction between these reactions is critical for understanding metabolic pathways and the energetics of cellular processes.