Because an endergonic reaction entails a gain of energy, that energy must be provided from an external source for the reaction to occur.
The reaction in which free energy is liberated is known as an exergonic reaction.
Protein synthesis is an anabolic process in which numerous amino acids are linked together to produce protein. Because this process requires energy, it absorbs it during the reaction. Because energy is only taken during endergonic reactions, protein synthesis is an endergonic reaction.