Final answer:
Enzymes are proteins that act as catalysts in chemical reactions, lowering the activation energy needed for the reaction to occur. They bind to reactant molecules and facilitate bond-breaking and bond-forming processes. Enzymes do not change the overall energy change of a reaction, but they significantly reduce the energy barrier.
Step-by-step explanation:
A substance that helps a chemical reaction to occur is a catalyst, and the special molecules that catalyze biochemical reactions are called enzymes. Almost all enzymes are proteins, made up of chains of amino acids, and they perform the critical task of lowering the activation energies of chemical reactions inside the cell. Enzymes do this by binding to the reactant molecules, and holding them in such a way as to make the chemical bond-breaking and bond-forming processes take place more readily.
It is important to remember that enzymes don't change the AG of a reaction. In other words, they don't change whether a reaction is exergonic (spontaneous) or endergonic. This is because they don't change the free energy of the reactants or products. They only reduce the activation energy required to reach the transition state.