Uptake of Elements from a Chemical Point of View

Abstract

In order to appreciate the initial link between the geosphere and the biosphere about 4 × 10⁹ years ago, certain characteristic, essential features of the organic chemistry of all known living organisms need to be noted and put into the context of the nature of primitive Earth. The chemicals concerned are in the cytoplasm, which is the central compartment of all cells (Fig. 5.1). The common features of cells of this cytoplasmic chemistry are the syntheses of polymers such as the production of lipids (fats), polysaccharides (sugars), proteins, and nucleotides (DNA and RNA). They are all products of a few non-metal elements extracted from the environment; namely carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus. Now the very nature of the polymers and their small molecule building blocks is that all of these six elements except phosphorus have to be in reduced states relative to CO(CO₂), H₂O, N₂, (O₂), and elemental sulfur. This means that they are bound not just to one another but are bound, except for phosphorus, mainly by hydrogen. There is then a requirement in all cells for the cytoplasm to have a reductive synthesis capacity (below −0.5 V). Because the required reductions cannot be brought about by organic chemicals, a transition metal ion redox catalyst, such as iron and copper, is also essential. This cytoplasmic chemistry is demanding in other ways because the initial small building blocks, such as simple aliphatic acids, sugars, amino acids, and bases, need to be polymerized. There is then a second requirement for a suitable Lewis acid/base catalyst and again metal ions, especially divalent ions magnesium and zinc, are extremely valuable in reactions such as condensation polymerization.

$$ \mathrm{XOH} + \mathrm{YH}=\mathrm{XY} + {{\mathrm{H}}_2}\mathrm{O} $$