A chemical reaction is a change brought about by the formation or breaking of chemical bonds. Two general types of reactions are synthesis reactions and decom-position reactions.
In a synthesis reaction, bonds are formed to join two or more atoms or molecules to make a new com-pound. The production of the protein hemoglobin in potential red blood cells is an example of a synthesis reaction. Proteins are synthesized by the bonding of many amino acids, their smaller subunits. Synthesis reactions require energy for the formation of bonds.
In a decomposition reaction, bonds are broken, and a large molecule is changed to two or more smaller ones. One example is the digestion of large molecules of starch into many smaller glucose mole-cules. Some decomposition reactions release energy; this is described in a later section on cell respiration.