Chemical evolution atmosphere

The influence of plant sterols on the phase properties of phospholipid bilayers has been studied by differential scanning calorimetry and X-ray diffraction [206]. It is interesting that the phase transition of dipalmitoylglycerophosphocholine was eliminated by plant sterols at a concentration of about 33 mole%, as found for cholesterol in animal cell membranes. However, less effective modulation of lipid bilayer permeability by plant sterols as compared with cholesterol has been reported. The molecular evolution of biomembranes has received some consideration [207-209]. In his speculation on the evolution of sterols, Bloch [207] has suggested that in the prebiotic atmosphere chemical evolution of the sterol pathway if it did indeed occur, must have stopped at the stage of squalene because of lack of molecular oxygen, an obligatory electron acceptor in the biosynthetic pathway of sterols . Thus, cholesterol is absent from anaerobic bacteria (procaryotes). 

Ronov, A. B. (1964). Common tendancies in the chemical evolution of the Earth s crust, ocean, and atmosphere, Geochem. 8,715-743. 

Hole MJ, Saunders AD, Marriner GF, Tamey J (1984) Subduction of pelagic sediments implications for the origin of Ce-anomalous basalts from the Mariana islands. J Geol Soc London 141 453-472 Holland HD (1984) The Chemical Evolution of the Atmosphere and Oceans. Princeton Uiuversity Press, Princeton... 

From the viewpoint of a model of prebiotic chemical evolution and of the primitive atmosphere of the Earth,174175 photosynthetic reactions of C02 were also examined, and formaldehyde with various nitrogen-containing products was obtained. 

If the primeval atmosphere did not contain enough CO2 to maintain a greenhouse climate, the much lower solar irradiation at that time would have led to frozen oceans. But that would make almost all the assumed synthetic mechanisms for the formation of biomolecules impossible Bada et al. (1994) consider external help as a way out of this dilemma. They assume that the energy from meteor impacts (diameters up to around 100 km), converted into heat, would have sufficed to melt the oceanic ice. If such a process were to have occurred periodically, chemical evolution reactions (see Chap. 4) could have taken place in the ice-free periods and have led finally to biogenesis. 

To model the chemical effects of evaporation, we construct a reaction path in which H2O is removed from a solution, thereby progressively concentrating the solutes. We also must account in the model for the exchange of gases such as CO2 and O2 between fluid and atmosphere. In this chapter we construct simulations of this sort, modeling the chemical evolution of water from saline alkaline lakes and the reactions that occur as seawater evaporates to desiccation. 

Holland HD (1984) The chemical evolution of the atmosphere and oceans. Princeton University... 

Thiemens MH, Heidenreich JE (1983) The mass independent fractionation of oxygen a novel isotope effect and its possible cosmochemical implications. Science 219 1073-1075 Thiemens MH, Jackson TL, Brenninkmeijer CAM (1995) Observation of a mass-independent oxygen isotopic composition in terrestrial stratospheric COj, the link to ozone chemisdy, and the possible occurrence in the Martian atmosphere. Geophys Res Lett 22 255-257 Timmes FX, Woosley SE, Weaver TA(1995) Galactic chemical evolution hydrogen through zinc. Astrophys J Suppl 98 617-658... 

Holland HD (1984) The Chemical Evolution of the Atmosphere and Oceans. Princeton University Press, Princeton... 

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. 

COMMENTARY OBSERVATIONAL ASPECTS RELATED TO THE CHEMICAL EVOLUTION OF OUR ATMOSPHERE... 

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. 

In summary, it can be stated that there is nothing markedly unusual about the Isua supracrustal facies and their organic contents. They can be compared closely with younger Precambrian, and in some cases Phanerozoic lithologies and successions. Depositional mechanisms and hydrosphere — atmosphere chemical equilibria appear to have been within the range of more recent times (Nutman et al., 1984)119). It seems from all indication that the advent of photoautrophy preceeded Isua times. It was probably the most crucial single event to make an impact on the evolution of the terrestrial atmosphere. 

However, before bacteria could evolve, the fundamental chemistry of life needed to be established. For this we need to turn back the clock to around 4.5 - 4.1 billion years ago where the earth s crust has cooled and solidified and the oceans and atmosphere begin to form. It is speculated that iron-sulfide synthesis along deep oceanic platelets may have lead to the synthesis of the first RNA and self-replicating molecules. Exactly how this chemical evolution came about remains an open question. It is possible that RNA may have used clays and similar self-replicating materials as substrates. Eventually, this... 

In recent years innumerable publications have dealt with the natural carbon cycle and its alteration by human activities. Some summary works of interest in this chapter are Atmospheric Carbon Dioxide and the Global Carbon Cycle (ed. Trabalka, 1985), The Carbon Cycle and Atmospheric CO2 Natural Variations, Archean to Present (eds. Sundquist and Broecker, 1985), Chemical Cycles in the Evolution of the Earth (eds. Gregor, Garrels, Mackenzie, and Maynard, 1988), History of the Earth s Atmosphere (Budyko, Ronov, and Yanshin, 1985), and The Chemical Evolution of the Atmosphere and Oceans (Holland, 1984). The interested reader is referred to these volumes for further discussion of material presented here. 

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