The energy released from the exergonic reaction can be used to drive the endergonic reaction. Endergonic reactions may also be 'pulled' forward by removing the products of the reaction as fast as they form, so that the reaction is prevented from reaching its equilibrium point. Chemical reactions, both spontaneous and non-spontaneous, require an initial input of energy, the energy of activation, to begin the reaction. This energy is needed to bring reactant molecules into position and condition for reaction. In non-living systems, this energy can be supplied by heat.
Enzymes serve as organic catalysts that lower the energy of activation, thus facilitating reactions.
The enzyme is able to interact with the reactants of a reaction to 'set them up' by distorting the molecules slightly, weakening their bonds and making it more likely that the molecules undergo a chemical change. Enzymes can also act to 'collect' molecules in a local area and to position them in such a way that bonds are easily formed between them. A basic goal of the respiratory pathways is to release the chemical energy of glucose or other nutrients and recapture that energy in the high energy bonds of ATP molecules. When glucose is fully oxidized, it is broken down to carbon dioxide and water. In the process, energy is released.