What type of reaction requires energy input to proceed and does not occur spontaneously?

An endergonic reaction is characterized by the requirement of energy input to proceed. In these reactions, the products have a higher free energy compared to the reactants, meaning that energy must be supplied for the reaction to take place. This concept is critical within the context of biochemical processes, where many metabolic reactions are not spontaneous and require adenosine triphosphate (ATP) or other forms of energy to facilitate the conversion of reactants into products.

In contrast, exergonic reactions release energy and tend to occur spontaneously, meaning they can proceed without the need for external energy input. Catalytic reactions involve the use of catalysts to speed up reactions but do not inherently determine whether the reaction is exergonic or endergonic. Spontaneous reactions are those that can proceed with the release of energy and, thus, do not require an input of energy, which further emphasizes the distinct nature of endergonic reactions in requiring energy for their progression.