Metabolism (from μεταβολισμος (metabolismos), the Greek word for "change"), is the biochemical modification of chemical compounds in living organisms and cells. This includes the biosynthesis of complex organic molecules (anabolism) and their breakdown (catabolism). Metabolism usually consists of sequences of enzymatic steps, also called metabolic pathways. The total metabolism are all biochemical processes of an organism. The cell metabolism includes all chemical processes in a cell.
Important metabolic pathways are:
Catabolic pathways that breakdown complex molecules into simple compounds:
Anabolic pathways that create building blocks and compounds from simple precursors:
Drug metabolism pathways, the modification or degradation of drugs and other xenobiotic compounds through specialized enzyme systems:
Nitrogen metabolism includes the pathways for turnover and excretion of nitrogen in organisms as well as the biological processes of the biogeochemical nitrogen cycle:
The first controlled experiments in human metabolism were published by Santorio Santorio in 1614 in his book Ars de statica medecina that made him famous throughout Europe. He describes his long series of experiments in which he weighed himself in a chair suspended from a steelyard balance (see image), before and after eating, sleeping, working, making love, fasting, depriving from drinking, and excreting. He found that by far the greatest part of the food he took in was lost from the body through perspiratio insensibilis (insensible perspiration).