Chemical bonds represent stored energy. When chemical bonds are formed or broken down, chemical reactions happen. During chemical reactions, energy can be released or absorbed. Exergonic reactions occur when more energy is released than absorbed. By contrast, endergonic reactions occur when more energy is absorbed than released. The main characteristic of metabolism is the coupling of exergonic and endergonic reactions. For example, energy can be released by exergonic reactions during which nutrients such as glucose are broken down. When glucose is broken down, the released energy can be used to produce up to 32 ATP molecules. Energy produced by ATP molecules can be used for endergonic reactions, during which body structures such as bones and muscles are built or movements in or out of cells are completed.