Exergy of Living Forms

The concept of exergy is so omnicomprehensive that the possibility of applying it to the analysis of living forms exerts a formidable appeal on biologists, physicists, and thermodynamicists. The problem is complex and not entirely clear. It is obviously strictly linked to the problem of calculating the rate of entropy generation of living beings. A concise and very readable statement of the enormous implications of these considerations can be found in Schrodinger's What Is life?, that was though written in 1945, before much of the modern work on nonequilibrium thermodynamics was published. In modern terms, the best formulation presently available is probably that of Kay and Schneider, which also contains a review of previous work in the field. Their position is that 'life' is a 'tool' invented by evolution to better exploit the energy available on Earth. From elementary living organisms (bacteria, protozoa) to plants to carnivore mammals, the 'goal' of living systems would be that of 'smoothing out gradients', capturing and degrading the incoming energy in the most 'efficient' way (i.e., with the smallest global entropy generation possible under the given boundary conditions). J0rgensen and co-workers elaborated on this approach, introducing the concept of 'growth of complexity' and of 'information content' which they analyze in terms of exergy. According to this view, a more complex structure, which contains more genetic information, may emerge from a less complex one without causing a global exergy destruction.

This approach is opposed by 'classical' thermodynami-cists; they maintain that, while it is true that Prigogine's idea that a structure more complex than its surroundings may 'release' entropy into its proximate environment and position itself at a lower entropic level, the conditions under which this phenomenon appears are not general, but strongly linked (and limited) by the specific physical characteristics ofboth the system and its environment and by the particular boundary conditions applied. In other words, they negate the general validity of the so-called 'Lotka principle', according to which a colony oforganisms always structures itself in such a way to maximize the absorbed power.

We shall not delve into this topic here: suffice it to say that it is a highly controversial issue, and that seemingly sound 'proofs' have been presented by both sides. Until a general time-evolution equation for nonequilibrium systems is found, the problem remains open.

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