Anabolic reactions, which are endogenic in nature, require energy for synthetic processors. The catabolic reactions which are exorganic in nature and give energy are coupled to anabolic reactions. This process of coupling maintains a balance in the living systems. The coupling of those reactions is an important characteristics of living organism and is carried out with the help of enzyme directed reaction pathways. This coupling process can be understood by examining a simple analogy where an energy releasing catabolic chemical reaction is represented by stones falling from a hill top. The kinetic energy of falling stones would normally be wasted in the form of heat generated when they hit the ground.
Part of this kinetic energy of falling stones can be conserved in the form of lifted bucket of water which can be compared to the synthesis of ATP and the conserved energy can be used for energy requiring reaction. Such as utilization of ATP during undergonic reactions. So the stones falling from the hill top can be compared with the oxidation of food stuff. In the living organisms, enzymes played the role of paddle wheels and generate and trap energy in the form of high energy phosphate bonds of ATP.