In biochemistry, metabolism (from Greek μεταβολή meaning “change”) is the set of chemical transformations that are dedicated to maintaining life within the cells of living organisms. These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures and respond to the stresses of the surrounding environment. The word “metabolism“ can also refer to all those chemical reactions that take place in living organisms, including the digestion and transport of substances within cells and between different cells, in which case the series of reactions that take place within the cells is called intermediate metabolism.
In the chemical transformations that continuously take place in the organism, two phases or rather two directions can be recognized: cleavage and degradation processes (disassimilation) through which cellular constituents and reserve substances are transformed into smaller molecules (catabolism); assimilation and synthesis processes, which allow the formation of new living matter or the accumulation of new reserve material in the cells (anabolism).
In the course of catabolic processes there is the liberation of energy, which is largely transferred to the environment in the form of heat or work, but a certain amount of energy is used for the reformation of other molecules, especially during development. Each phase of metabolism must therefore be examined under the aspect of both chemical and energy; almost always, in fact, the biological significance of the chemical compounds involved in metabolic transformations can be seen in the energy phenomena that take place as a result of these transformations.
The energy used by living organisms is the potential energy, of chemical nature, contained in some organic constituents of tissues. Cells consume this energy incessantly, which would end up running out if it were not renewed continuously. This is provided by the nutritive activity, that is the use of chemical energy provided by organic molecules assimilated by the environment. These molecules (carbohydrates, proteins, lipids) are therefore real biological fuels: in the animal organism they are demolished, in the presence of oxygen, to carbon dioxide and water. This process, which occurs through hundreds of intermediate chemical reactions, involves many energy transformations and the synthesis of intracellular reserve materials.
Autotrophic organisms behave differently (see chlorophyll photosynthesis). In higher animals, the elementary nutrients formed by the digestion of carbohydrates, proteins and triglycerides (i.e., glucose, fatty acids, amino acids, etc.) retain the intrinsic energy content of the original substances unaltered.
For the production of energy, however, it is necessary that these molecules are completely demolished in the tissues. This happens with the intervention of specific oxidoreductive enzymes, which transform the molecules of glucose, fatty acids and amino acids in smaller fragments, until the formation of a compound with two carbon atoms, the metabolite acetyl-CoA. This complex of metabolic transformations constitutes intermediate metabolism. At the end of intermediate metabolism about one-third of the energy contained in the starting materials is made available to the cells. The remaining two thirds are released and used during a subsequent series of cyclic metabolic reactions (Krebs cycle), also catalyzed by oxidoreductive enzymes. Through these reactions acetyl-CoA is completely degraded to the formation of carbon dioxide and water (terminal metabolism).
The set of energy transformations that occur in the various stages of metabolism is defined as energy metabolism: in higher animals the energy transformed by the body should correspond to the chemical energy contained in food. This energy is used mainly in the form of heat (to maintain a stable body temperature at 37 ºC), and in the form of mechanical energy, necessary for muscle work, for osmotic and electrical exchanges and for the performance of cellular endoergonic synthesis.
To calculate the energetic metabolism of an individual one can then proceed in two ways: measure the energetic value of food; measure the amount of heat produced, when both mechanical work and any other form of energy developed by the organism have been transformed into thermal energy. The validity of the latter method derives from the fact that in a closed system the total amount of energy remains constant, despite all the transformations that may occur in it. In other words, the various forms of energy are equivalent to each other, so it is possible to calculate the total amount of energy released by the organism without taking into account the different forms in which this energy is released. In fact, if the organism does not perform work, almost all metabolic energy is given up in the form of heat. For this reason energy metabolism is generally measured as the amount of heat and the unit of measure used is the large calorie (Cal).
It should be noted, however, that the equivalence between the energy introduced with food and the one given to the environment does not exist when a part of the energy introduced is accumulated in the tissues in the form of chemical bonds with a high dynamic-energetic content or when a part of the energy given to the environment comes from the scission of these bonds.