A standard textbook in biology states to sustain the processes of life, a typical cell carries out thousands of biochemical reactions each second. The sum of all biological reactions constitutes metabolism. What is the purpose of these reactions - of metabolism? Metabolic reactions convert raw materials, obtained from the environment, into the building blocks of proteins and other compounds unique to organisms. Living things must maintain themselves, replacing lost materials with new ones; they also grow and reproduce, two more activities requiring the continued formation of macromolecules. (Purves et al. 1992, p. 113)
Metabolism is the totality of the biochemical reactions in a living thing. These reactions proceed down metabolic pathways, sequences of enzyme catalyzed reactions, so ordered that the product of one reaction is the substrate for the next. Some pathways synthesize, step by step, the important chemical building blocks from which macromolecules are built, others trap energy from the environment, and still others have functions different from these. (Ibid., p. 130)
The term 'metabolism' (Stoffwechsel) was introduced as early as 1815 and was adopted by German physiologists during the 1830s and 1840s to refer primarily to material exchanges within the human body, related to respiration. Justus Liebig then extended the use of the term to the context of tissue degradation, in combination with the somewhat mystic notion of 'vital force' (Liebig 1964 ). By contrast, Julius Robert Mayer, one of the four co-discoverers of the law of conservation of energy, criticized the notion of 'vital force' and claimed that metabolism was explicable entirely in terms of conservation of energy and its exchange (Mayer 1973 ). Later this term was generalized still further and emerged as one of the key concepts in the development of biochemistry, applicable both at the cellular level and in the analysis of entire organisms (Bing 1971).
Whereas the concept of metabolism was and still is widely applied at the interface of biochemistry and biology when referring to cells, organs and organisms, it became a matter of dispute whether it is applicable on any level further up the biological hierarchy. E.P. Odum clearly favors the use of terms like 'growth' or 'metabolism' on every biological level from the cell to the ecosystem (for example, Odum 1959). Which processes may and should be studied on hierarchical levels beyond the individual organism, though, is a matter of debate dating back to Clements (1916) and still going on.
Tansley (1935) established 'ecosystem' as a proper unit of analysis. He opposed Clements' 'creed' of an organismical theory of vegetation. Lindemann (1942) analyzed ecosystems mathematically, with plants being the producer organisms to convert and accumulate solar radiation into complex organic substances (chemical energy) serving as food for animals, the consumer organisms of ecosystems. Every dead organism then is a potential source of energy for specialized decomposers (saprophagous bacteria and fungi) thereby closing the cycle. This is in essence what Odum referred to when talking about the metabolism in an ecosystem.
Basically this is a debate about 'holism' (or organicism) v. 'reductionism'. Do populations (that is, the interconnected members of a species), communities (the total of living organisms in an ecosystem) or ecosystems (the organisms and the effective inorganic factors in a habitat) have a degree of systemic integration comparable to individual organisms? Does evolution work upon them as units of natural selection? These questions are contested in biology, and thus a use of the term 'metabolism' for a system consisting of a multitude of organisms does not pass unchallenged.
Like any other animal, humans are heterotrophic organisms, drawing energy from complex organic compounds (foodstuff) that have been directly or indirectly synthesized by plants from air, water and minerals, utilizing the radiant energy from the sun. But humans as a species are not able to survive and maintain their metabolism individually. Does it make sense, therefore, to look at human communities and societies in terms of entities of cooperatively empowered metabolism? Societies will be bound to have collective metabolism that is, at least, the sum of the individual metabolisms of its members. If a society cannot maintain this metabolic turnover, its population will die or emigrate. But not all materials need to be processed through the cells of human bodies. From an ecosystem perspective, for example, the materials birds use in building their nests constitute a material flow associated with birds. In ordinary biological language, however, this is not part of bird metabolism, although it may be vital for bird reproduction. Thus the concept 'metabolism' needs to be expanded to encompass material and energetic flows and transformations associated with 'living things' but extending beyond the anabolism and catab-olism of cells. The overall material and energetic turnover of an ecosystem component, its consumption of certain materials, their transformation and the production of other materials, may be an ecologically useful parameter. In biology this would not be called metabolism.
Humans, of course, sustain at least part of their metabolism not by direct exchanges with the environment (as, for example, in breathing), but via the activities of other humans. This is a matter of social organization. Any attempt to describe this organization in terms of a biological system - whether it be the organism, or a population in a habitat, or an ecosystem - draws on analogies and runs the risk of being reductionist.2 On the other hand, the concept of metabolism in biology has valuable features: It refers to a highly complex self-organizing process which the system seeks to maintain in widely varying environments. This metabolism requires certain material inputs from the environment, and it returns these materials to the environment in a different form.
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