Chemical evolution complex systems

The photochemistry of the polluted atmosphere is exceedingly complex. Even if one considers only a single hydrocarbon pollutant, with typical concentrations of nitrogen oxides, carbon monoxide, water vapor, and other trace components of air, several hundred chemical reactions are involved in a realistic assessment of the chemical evolution of such a system. The actual urban atmosphere contains not just one but hundreds of different hydrocarbons, each with its own reactivity and oxidation products. 

A typical chemical system is the oxidative decarboxylation of malonic acid catalyzed by cerium ions and bromine, the so-called Zhabotinsky reaction this reaction in a given domain leads to the evolution of sustained oscillations and chemical waves. Furthermore, these states have been observed in a number of enzyme systems. The simplest case is the reaction catalyzed by the enzyme peroxidase. The reaction kinetics display either steady states, bistability, or oscillations. A more complex system is the ubiquitous process of glycolysis catalyzed by a sequence of coordinated enzyme reactions. In a given domain the process readily exhibits continuous oscillations of chemical concentrations and fluxes, which can be recorded by spectroscopic and electrometric techniques. The source of the periodicity is the enzyme phosphofructokinase, which catalyzes the phosphorylation of fructose-6-phosphate by ATP, resulting in the formation of fructose-1,6 biphosphate and ADP. The overall activity of the octameric enzyme is described by an allosteric model with fructose-6-phosphate, ATP, and AMP as controlling ligands. 

M. T. Beck, Prebiotic Coordination Chemistry The Possible Role of Transition Metal Complexes in the Chemical Evolution , in Metal Ions in Biological Systems , ed. H. Sigel, Dekker, New York, 1978, vol. 7. 

In addition to the transition phenomena mentioned so far in the present section, a variety of even larger scale processes might have operated during chemical evolution, namely, instabilities and bifurcations in the very atmospheric environment within which life emerged. As shown in the paper by Marcel Nicolet, the earth s atmosphere is the theater of a variety of complex chemical and transport phenomena. Moreover, as explained by Stanley L. Miller, the composition of the primordial atmosphere has certainly affected deeply the chemistry in the primitive oceans. Conversely, once life emerged the properties of the atmosphere changed radically, and this must have affected the further course of evolution. We refer to Prather et al.41 and North et al.42 for an account of present views on large scale transitions in the earth-atmosphere system. 

It is important to notice that this is very different from the mechanism of chemical evolution. Kauffman and Dyson, it will be remembered, have shown that the probability of a spontaneous transition from chaos to order increases with the complexity of the system, but in this case the order (or antichaos) is not a result of natural conventions and has nothing to do with organic codes. 

In tHe course of biological evolution of Hving systems, tHere could be indeed a simultaneous compHcation of the network of biochemical trans formations of the substances constituents of metaboHc cycles as weU as an increase of these transformations in number. The characteristic times of the elementary chemical transformations of evolutionary precursors remained typicaUy unvaried. However, while the system compHcation causes an increase in the total number of the individual stages, the total time for accomplishing the entire metaboHc cycle is increasing. That led to the conclusion that more complex systems must have lower rates of energy dissipation P at the same point in the cycle, and, therefore, they must be treated as more perfect from the point of the biological evolution. 

The final topic upon which 1 touch is far from the center of the current conference, but central to the evolution of the biosphere, this overwhelmingly complex chemical non-equilibrium system. 

The most recent computations of chemical reaction paths couple chemical kinetics, path calculations, and fluid flow models. This can be accomplished by alternating between fluid flow and reaction path calculations in small time steps, with reaction kinetics included as we have described above. Several examples of this type are summarized by Brimhall and Crerar (1987, pp. 302-306). With this kind of approach it should ultimately become possible to model the detailed physical and chemical evolution of quite complex natural mineral systems. With inclusion of three-dimensional space as well as temperature and pressure gradients, there are challenges for the foreseeable future. 

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