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.
Exothermic reactions are often spontaneous.
By contrast, endothermic reactions are rarely spontaneous because they need an energy input. Once started, however, the reaction liberates more energy than the amount initially added. For example, table sugar does not burn when exposed to air. However, if energy is added by holding a flame under the sugar, it will begin burning and will ultimately release more energy than was needed to start the reaction. When you add heat to the reactants the energy level of the reactants is increased.