Metabolism (from μεταβολισμος ("metabolismos")) 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. Without metabolism we would not be able to survive.

The term is derived from the Greek word for "change", or "overthrow".

Metabolic pathways

Important metabolic pathways are:

General pathways

• Carbohydrate metabolism
• Fatty acid metabolism
• Protein metabolism
• Nucleic Acid metabolism

Catabolism

Catabolic pathways that breakdown complex molecules into simple compounds:

• Cellular respiration, metabolic pathways that create energy (ATP and NADPH) from fuel molecules. These pathways are also involved in the digestion of food.
  • Carbohydrate catabolism
    • Glycogenolysis, the conversion of glycogen into glucose.
    • Glycolysis, the conversion of glucose into pyruvate and ATP, does not require oxygen.
      • Embden-Meyerhof pathway, the common glycolysis pathway.
      • Entner-Doudoroff Pathway, an alternative glycolysis pathway in few bacteria.
    • Pentose phosphate pathway (hexose monophosphate shunt), generation of NADPH from glucose.
  • Protein catabolism, the hydrolysis of proteins into amino acids.
• Aerobic respiration
  • Electron transfer chain
  • Oxidative phosphorylation
• Anaerobic respiration,
  • Cori cycle
  • Lactic acid fermentation
  • Fermentation
  • Ethanol fermentation

Anabolism

Anabolic pathways that create building blocks and compounds from simple precursors:

• Glycogenesis
• Gluconeogenesis
• Porphyrin synthesis pathway
• HMG-CoA reductase pathway, leading to cholesterol and isoprenoids.
• Secondary metabolism, metabolic pathways that are not essential for growth, development or reproduction, but that usually have ecological function.
• Photosynthesis
  • Light-dependent reaction (light reaction)
  • Light-independent reaction (dark reaction)
• Calvin cycle
• Carbon fixation

Drug metabolism

Drug metabolism pathways, the modification or degradation of drugs and other xenobiotic compounds through specialized enzyme systems:

• Cytochrome P450 oxidase system
• Flavin-containing monooxygenase system
• Alcohol metabolism

Nitrogen metabolism

Nitrogen metabolism includes the pathways for turnover and excretion of nitrogen in organisms as well as the biological processes of the biogeochemical nitrogen cycle:

• Urea cycle, important for excretion of nitrogen as urea.
• Biological nitrogen fixation
• Nitrogen assimilation
• Nitrification
• Denitrification

Other

• Human iron metabolism

History

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, sex, 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).