In an isolated system, energy can be changed from one form to another and transferred from one molecule to another, but energy cannot be created or destroyed . The 2 nd Law of Thermodynamics states that energy conversions reduce the order in the universe. That reduced order is called entropy . Heat is one form of disorder or entropy. They regulate chemical reactions, through metabolism.
Metabolism is the sum total of all chemical reactions in a cell.
These reactions have a net release of energy so they are called exergonic reactions. Examples of catabolic reactions would be hydrolysis and the reactions involved in cellular respiration. There is a net input of energy so these reactions are called endergonic . Products of endergonic reactions have higher potential energy. Examples would be biosynthetic reactions and dehydration synthesis.