Final answer:
Endergonic reactions are less likely to occur than exergonic reactions because they require an input of energy and do not occur spontaneously. Enzymes can speed up both types of reactions but cannot change the fundamental nature of the reaction from endergonic to exergonic.
Step-by-step explanation:
All else being equal, endergonic reactions are less likely to occur as exergonic ones. Endergonic reactions require an input of energy as they proceed with a positive free energy change, meaning that they do not occur spontaneously under standard conditions. Energetic and enzymatic considerations highlight the differences between these reactions.
Both types of reactions require overcoming an activation energy barrier. However, this activation energy is always positive, whether the reaction is endergonic or exergonic. The transition state of a reaction is always at a higher energy level than the reactants, not a lower one.
While enzymes can accelerate reactions by lowering the activation energy, it's essential to note that they cannot convert an endergonic reaction into an exergonic one. The distinction between reaction types relies solely on the intrinsic energy differences between the reactants and products, which is determined by their chemical nature.