Life on the planet earth is a very complex process of biochemical reactions of various elements .human body has the ability to control and change its internal environment which is called Milieu Interior. Milieu Interior is a very complex biochemical and biophysical system. Of the trillions of cells that make up our body, not a single cell can survive alone because every cell and every molecule that makes it depends on others for its functions Due to this system the human body has developed the ability to adapt to the environment both internal as well as external. There is always an energy requirement in the human cells. They take in digested food to either produce energy for its various functions or to make new molecules. This process of chemical reactions of break down and synthesis as a whole is called Metabolism which is the topic of this book.