All organisms require energy to live and to carry out their life functions. ATP, the energy currency of organisms, is obtained from the oxidation of energy-rich organic compounds. Since vertebrates cannot synthesize these organics from inorganic sources, they must obtain them by eating other organisms. Vertebrates have evolved highly efficient digestive systems to process their food. Various chambers along the one-way tract are specialized for functions such as obtaining the food, mechanically breaking the food into small pieces, temporarily storing it, using acids and enzymes to break it into molecular fragments, absorbing the small building-block molecules, and passing the unabsorbed waste materials outside the body. Our digestive systems process our food – our fuel – and make it available to be utilized by our cells. Without such a system our cells, and thus ourselves, would starve to death. In processing our food, our digestive systems also provide us with the necessary raw materials to make new macromolecules, cells, and tissues.