Chemistry Reference
In-Depth Information
We now discuss just why these elements out of the entire periodic table have been selected. One thing is clear
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they were not only selected as a function of their abundance and their availability in the universe as well as in the
earth's crust, and the oceans (which constitute the major proportion of the earth's surface), but also on the basis of
their suitability for the functions that they are called upon to play, in what is predominantly an aqueous
environment.
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It therefore comes as no great surprise that within our solar system itself, all 11 of the principal elements found
in man are in the top 20 in terms of abundance, with five of them figuring in the top ten
hydrogen, carbon,
nitrogen, oxygen, and sulfur. When we consider the abundance of these 11 obviously essential elements in the
earth's crust (
Fig. 1.2
), we find that no less than six of them (hydrogen, oxygen, and the four alkali and alkaline
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FIGURE 1.2
Abundance (atom fraction) of the chemical elements in Earth's upper continental crust as a function of atomic number.
earth metals cited above
sodium and potassium, magnesium, and calcium) are among the top ten (together with
aluminium, silicon, titanium and, not surprisingly, iron, since the earth's core is predominantly constituted by iron,
together with significant amounts of nickel). The remaining five are among the top 20.
But we have every reason to believe that life, as we know it, originated from the oceans, so we also need to
consider the distribution of the eleven essential elements in this environment. This is, of course, influenced by
the solubility of the corresponding element in salt water. So, it is no surprise that today we find very low
concentrations of iron in the oceans (although, if the primitive atmosphere was, as we think, reducing, divalent
ferrous iron would have been readily available in a soluble form). So, of our eleven key elements, how many
are now found in the water of our oceans? Clearly, sodium and chlorine for starters, but hydrogen, oxygen, and
carbon, together with magnesium, sulfur, calcium, potassium, and bromine, make the top 10. The only two
which do not make it are nitrogen and phosphorus, and we know that they also exist in non-negligible
amounts.
So, of the 11 principal elements that are found to be essential for human life, they are all omnipresent in the
solar system, the earth's crust, and the oceans. Of course, they would have had to be 'bioavailable' (a designation,
which really means not just being in the right place at the right time, but being free to be assimilated by the
biological system in question).
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4. Another important distinction between organic chemistry and the chemistry of living organisms (biochemistry) is that the former is carried
out almost entirely in nonaqueous media, whereas the latter occurs essentially in approximately 56 M H
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O.
