The absorption of digested food is followed by assimilation. The assimilated food undergoes innumerable chemical changes before it can be utilized to meet the various needs of the body.
Amino acids are used for protein synthesis in the body and also for maintaining and repairing of old worn-out tissues. Some of the amino acids are de-aminised (removal of NH2, group) and the rest of them used for energy purposes. Fatty acids and glycerol provide energy to the body, while a part of these is stored as fats in the adipose tissues. Similarly, the simple sugars are used for energy and the excess is converted into glycogen. Glycogen is stored in the liver and muscles.
The human body is like a machine, experiencing a certain amount of wear and tear daily (breakdown process). The daily loss is replenished or made good by constructive processes to maintain proper functioning of the body.
The breakdown and the synthesis activities that go on simultaneously in the body are collectively called metabolism. Metabolism refers to all chemical reactions resulting in catabolic (breakdown) and anabolic (synthesis or constructive) activities. Catabolism refers to all chemical reactions involved in the breakdown of body materials. It can be defined as a process by which energy is obtained by the oxidation of body constituents.
Energy thus released is utilized for the building up processes. Anabolism can be defined as a process by which energy released due to catabolic activities is utilised for the constructive and synthetic processes of the body.