1. Much of our food is not directly usable by cells. These substances, manufactured by other organisms, are not useful to cells until they are broken down and reassembled into molecules that the cell can then use.

2. Large macromolecules must first be broken down to their smaller subunits to first facillitate their passage into the blood and eventually their movement into the cells. Once at the cells, these subunit molecules may be reassembled back into the macromolecules needed by the cell.

The 3 main types of food molecules that must be digested into smaller components include proteins, starches and the triglycerides (fats and oils). The digestive system breaks these macromolecules down at body temperature. These reactions would normally occur very slowlly, but the digestive system secretes specific enzymes that speed the process up.

[Examples of Enzymes the Body Utilizes for Digestion]