The presence of energy is revealed only when change occurs. The energy transformation from one form to another follows the law of the conservation of energy. The glycolytic system uses only the energy stored in carbohydrate molecules such as glucose or glycogen for replenishing the ATP which the cell needs. The free energy yielded from splitting of the phosphate bond of ATP powers all forms of biological work. The glycolytic pathway, which only applies to carbohydrates, the aerobic pathway allows oxidation of not only carbohydrates but also lipids and proteins. Protein contains energy, but contributes little to energy metabolism. Both the nervous and endocrine systems often work together as part of a control system. They are structured so as to be able to sense information, organize an appropriate response, and deliver the message via neurotransmitters or hormones to the proper organ or tissue in order to exert their actions.