Metabolism

n. process in which food is converted to provide energy and to produce and maintain cells and tissues (Physiology, Chemistry)

Metabolism is the complete set of chemical reactions that occur in living cells. These processes are the basis of life, allowing cells to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories. Catabolism yields energy, an example being the breakdown of food in cellular respiration. Anabolism, on the other hand, uses this energy to construct components of cells such as proteins and nucleic acids.
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(n.)
The act or process, by which living tissues or cells take up and convert into their own proper substance the nutritive material brought to them by the blood, or by which they transform their cell protoplasm into simpler substances, which are fitted either for excretion or for some special purpose, as in the manufacture of the digestive ferments. Hence, metabolism may be either constructive (anabolism), or destructive (katabolism).
  

The whole range of biochemical processes that occur within us (or any living organism). Metabolism consists both of anabolism and catabolism (the buildup and breakdown of substances, respectively).

The total of all chemical changes that take place in a cell or an organism. These changes produce energy and basic materials needed for important life processes.