Prebiotic Chemistry

The Path from Inorganic to Organic Matter is Mediated through Water

Our notion is that water is the fundamental molecule of life, and that it is a necessary component for all known life forms and their sustainability [2]. Water molecules in solid liquid or gas forms are abundant on Earth. The total volume of water reaches the staggering level of 1.4 billion km3, with 97% of this amount stored in oceans. Water was one of the primordial elements of Earth’s environment and its atmosphere, alongside carbon dioxide (CO2), nitrogen and hydrogen. There are questions regarding the source of water, and a possible explanation is bombardment of our primordial and still- under-formation planet by extra-terrestrial objects such as meteorites. This hypothesis is supported by various findings showing that some meteorites may contain <20% of water [3]. Also, oxygen itself is one of the most abundant elements in the Earth’s crust.

A comprehensive outline of the abundance of various elements is shown in Figure 1.1. Besides water, living systems and life forms are based essentially on carbon (more specifically, carbon that is part of hydrocarbon molecules). These ‘organic’ molecules are immiscible with water. Thus, the functions of life forms are based on an interplay and cooperation between these systems. This cooperation involves an exchange of mass, electron charge, and energy between essentially two-phase systems. Water is a highly polar solvent, and this polarity enabled the spontaneous formation of ‘micro machines’ such as membranes. The solvation of inorganic salts and minerals and their high heat capacity and heat of vaporisation are crucial for stabilisation of the temperature in the system. This stabilisation and preservation of the chemical environment and metabolism in a closed living system is called ‘homeostasis’.

Percentage distribution of the most abundant elements in the Earth’s crust

Figure 1.1 Percentage distribution of the most abundant elements in the Earth’s crust. Adapted from J.E. Andrews, P. Brimblecombe, T.D. Jickells and P.S. Liss in An Introduction to Environmental Chemistry, Blackwell Science Ltd, Hoboken, NJ, USA [1]

The factors mentioned above have raised philosophical questions on why we need to continue seeking the existence of extra-terrestrial life based on the presence of aqueous environments on other planets. For example, does the fact that life on Earth is based on water mean that water is the universal molecular of life? Can another type of molecule under different temperature, pressure and chemical compositions have this role provided that ‘ideal’ circumstances may be met? The main transitions in physical properties for a series of small and very common molecules that have been proposed to be candidates as alternative solvents compared with water (melting point, 0 °C, boiling point, 100 °C) are: ammonia (-78 °C, -33 °C); methane (-182 °C, -164 °C); acetylene (-183 °C, -89 °C); and methanol (-94 °C, 65 °C).

 
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