Prebiotic matter

Cl chondrites contain 6% of their cosmic complement of carbon, mainly in the form of organic matter. The intense controversy that once surrounded the origin of this organic matter has subsided. Most authors now agree that this material represents primitive prebiotic matter, not vestiges of extraterrestrial life. The principal questions remaining are what abiotic processes formed the organic matter, and to what extent these processes took place in locales other than the solar nebula interstellar clouds or meteorite parent bodies. 

Our solar system and the Earth are thought to be about 4.6 Ga (4.6 billion or 4.6 x 10 years) old (Fig. 1). Meteorite bombardment made the surface of our planet a rather inhospitable place during the first few hundred years after its formation, however, isotopic patterns in sediments from Greenland indicate that life already existed as early as 3.8 Ga ago. The upper limit for the emergence of life is set by the intense bombardment of the Earth between 3.9-4.1 Ga ago, which without doubt sterilized it, e.g., by boiling off the oceans. " destroying the prebiotic matter... 

Calculation of the internal cell potential is a very complicated matter because the electrochemistry of all of the species within the protocell would have to be balanced subject to their composition quotient Q, after which the standard free energy would have to be established from tabulations. The transport of Na+ would also change this balance, along with the ionic strength of the solution and the stability of the proteins or prebiotic molecules within the protocell. Such non-equilibrium thermodynamics forms the basis of the protocell metabolism. The construction... 

In other words, since we do not know, each of us is free to choose. Do as you wish so long as you show that it is possible, respecting the prebiotic conditions, to create life from inanimate matter. This is the challenge and the method is open-ended. The ambition of scientists working in the field would be simply to arrive at minimal life a system containing the minimal and sufficient molecular ingredients to be called alive (this notion will be discussed in detail later on in this book). Of course this also calls into question the definition of hfe, a difficult issue but not an unsolvable one, as we will also see in the next chapter. 

To say that the natural laws may have governed the prebiotic scenario and all that happened in terms of reactivity and transformations, is one thing. To say that the natural laws have constructed a series of causal steps to lead to life, is another matter in fact, the latter assumes that the determiifism is purposely guided towards the formation of life. The natural laws per se do not have a preferential direction,... 

More generally, I will try to illustrate the different views on the origin of life and early evolution - notions like determinism and contingency will come into focus. All these scientific views are based on the postulate that life on Earth comes from inanimate matter and a corollary of this postulate is that it might be possible to reconstitute life in the laboratory, at least in some elementary form. The ambition of understanding the prebiotic chemistry leading to the transition to life, and ultimately, to the Faustian dream of making hfe on the workbench, underlies the whole field -and is also the common thread of this book. 

One of the more difficult prebiotic syntheses is that of the nucleosides. Heating of ribose and purines at 100°C gives fair yields (2-20%) of a mixture of isomers of the purine ribosides, with some of the correct 13-isomer being produced.45 However, a mixture of pyrimidines and ribose gives no detectable yield of nucleosides. Whether the inability to synthesize pyrimidine ribosides is a matter of not trying the correct conditions or whether pyrimidine nucleosides were not involved in the first organisms remains to be determined. 

Those facts generate the central problem in prebiotic chemistry. Spontaneous self-organization is not known to be an intrinsic property of most organic matter, at least as observed in the laboratory. It can be driven only by an external source of free energy that is coupled to the organic system. 

Therefore, gas phase photochemistry of inorganic and simple organic gases may have contributed to the prebiotic synthesis of organic materials, which subsequently may have been building blocks for animate matter [19]. 

The configuration of natural amino acids has led to studies on the possibility that homochirality emerged at a prebiotic stage, which may be supported by the presence of amino acids as non-racemic mixtures in meteorites (32). This enantiomeric excess may have resulted from the exposition of extraterrestrial matter to circularly polarized light (42). Whatever the origin of this enantiomeric excess, it may have initiated stereoselective processes through different catalytic pathways (7). Symmetry breaking may also have resulted from reactivity in connection with other processes such as crystallization or interfacial chemistry (43, 44) and polymerization of amino acids (45). 

The evolution of life required a series of transitions, beginning with the generation of organic molecules that could serve as the building blocks for complex biomolecules. How these molecules arose is a matter of conjecture, but experiments have established that they could have formed under hypothesized prebiotic conditions. 

A final aspect to the fitness of proteins relates to their evolutionary accessibility, especially in the early stages of cellular evolution, that he close to the origin of life. Henderson makes the point that in the case of water and carbonic acid In places where life is possible the primary constituents [water and carbonic acid] are necessarily and automatically formed in vast amounts by the cosmic process (1913, p. 268). Intriguingly, many of the twenty proteinaceous amino acids - out of which the folds are constructed - are among the commonest amino acids foimd in meteorites and the easiest amino acids to generate in prebiotic syntheses (Denton et al., 2002). Remarkably, the folds - so fit in so many other ways - are not so many steps removed from the synthesis of the elements in the stars, a witness to the fitness of matter for the evolution of protein-based life. 

Bifidobacterium counts in the cecum of rats compared to a control diet, but there was no significant difference in Lactobacillus coimts (Kuda et ah, 2005). Studies carried out on seaweed extracts found that fucoidan also functions as a good prebiotic. Several other studies have also confirmed the positive dietary effects of alginates encouraging the growth of beneficial microbial faima in fecal matter (Wang et ah, 2006). 

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