All organisms live by performing a wide variety of chemical reactions.
Respiration, photosynthesis, protein synthesis and DNA replication are all processes which involve multiple chemical reactions. Metabolism is the sum total of all chemical reactions performed by an organism. The energy of the products usually differs from that of the reactants. In exothermic reactions such as the oxidation of carbohydrates the energy of the products is less than that of the reactants, therefore energy is released. Conversely, endothermic reactions such as the formation of carbohydrates by photosynthesis require energy for the reaction to proceed because the energy of the products is greater than that of the reactants.