Prebiotic atmosphere

The prebiotic atmosphere had reducing properties, so that the bioelements C, O, N and S were present in reduced form as CH4, H2O, NH3 and traces of H2S. 

New experiments using weakly reducing or neutral gas atmospheres were conceived and carried out more than 20 years after Miller s first successes (Schlesinger and Miller, 1983). Comparisons of a series of simulated prebiotic atmospheres containing CH4, CO and CO2 as carbon sources, using electrical discharges at 298 K, led to the following results ... 

To investigate the role of water photolysis in ozone production in a prebiotic atmosphere, the following mechanism can be explored ... 

Calculate the dissolved concentrations of N2 and CO2 for a prebiotic atmosphere with a species ratio of 90 10 and a total pressure of 1 bar. 

Electricity has generally been recognized as a probable promoter of organic synthesis going back billions of years. A report by Sutherland and Whitfield summarizes some of the evidence for this.82 Thus, electrical spark discharges in the presence of simple molecules believed to be present in the planet s prebiotic atmosphere have been shown to produce a few of the building blocks needed for the creation of life. The sun also provided other forms of energy (e.g., infrared and ultraviolet radiation, inter alia). Some of the processes proved by laboratory experiments are ... 

Release of H from water might be effected by quite a number of different 3d-ions via the hydridocomplexes although CO and cyanide appear to be most simple and thus primitive ligands, it remains doubtful whether these enzymes are as archaic as often purported, given both the complexity of cyanide biosynthesis pathways in plants or fungi (Castric 1977 Gleadow et al. 2003) or arthropods and the difficulties which arise when substantial amounts of CO are assumed in the prebiotic atmosphere (cp. Chapter 5). 

Purine derivatives are considered to be prebiotic molecules. Purine as well as adenine (HCN) can be formed from hydrogen cyanide in the presence of ammonia in a prebiotic atmosphere. Similar reaction in the presence of water can lead to other purines. Purine chemistry goes back to the roots of organic chemistry. Uric acid was the first purine isolated from bladder stones by Scheelc in 1776. The correct empirical formula of uric acid was established much later by Liebig and Mitscherlich. The correct structure was suggested by L. Medicus and was shown to be correct by the syntheses ofHorbaezewski. Behrend and Roosen, and unambiguously by E. Fischer. The development of purine chemistry by E. Fischer is a milestone in heterocyclic chemistry. 

Box 2. The Content of CO2 in the Prebiotic Atmosphere, after Fenchel et al, 1998 Perhaps the most interesting question on the early atmosphere is related to the abundance and oxidation states of carbon. Current models predict a CO2-rich atmosphere, which would contain a trace amount of methane and a slightly greater amount of carbon monoxide. We can try to explain it on the basis of the following speculations. It is known that even today volcanic activity is a dominant process in releasing CO2 and it should have been so in the past. It is related to the reasonable suggestion that the oxidation state of the upper mantle was about the same as today. Furthermore, the by-products of water vapor photolysis were enabled to oxidize both CH4 and CO. The possible reaction pathway could be the following... 

For several decades, Oparin s model remained uncontradicted, the more so because experiments simulating prebiotic atmospheric conditions and applying electric discharges, ionizing particles, or UV-radiation were able to synthesize amino acids, small peptides, sugars, heterocyclic compounds and other biochemical molecules. 

Discuss the general items and steps in the atmosphere evolution with main attention to the content ofC02 in the prebiotic atmosphere and its alteration. 

The flux of solar ultraviolet (U V) radiation was one of the important environmental parameters on the early Earth. Present your understanding of this phenomenon and describe the main chemical processes in the prebiotic atmosphere. 

Just over fifty years ago, a major breakthrough occurred the celebrated Miller spark tube experiment (Miller, 1953), in which sparks (simulating electric storms) in a putative prebiotic atmosphere of methane, ammonia, dihydrogen, and water produced, under recycling conditions, a complex mixture of products containing in particular a variety of a-amino acids (identical to those used by all living organisms) (Miller, 1953). 

Photochemical reactions have played a decisive role in the evolution of the atmosphere and of life on Earth. Such processes generally involve simple species, many of which are otherwise considered to be stable and unreactive. Special Topic 6.17 has already discussed photochemistry in a prebiotic atmosphere. Since geochemical inorganic processes cannot be responsible for the current level of oxygen in the atmosphere (21 %), it must be almost exclusively the product of biological activity (Special Topic 6.25). 

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). 

Field B.O. and Speneer J.E.D. (1990) The synfliesis of amino acids and sugars on an inorganic template from constituents of the prebiotic atmosphere. Origin of Life and Evolution of Biosphere, 20,233-248. 

By using Stanley Miller s apparatus, it became possible to prove that the building blocks of life had their origin in the prebiotic atmosphere. 

Photochemistry of the prebiotic atmosphere (Dondi, Merli and Zeffiro, 41, 342-359)... 

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