The chemical elements essential to life

How many of the naturally occurring elements are essential to life? After more than a century of investigation the question still cannot be answered with certainty.

Only some time ago the best answer would have been twenty. Since then four more elements have been shown to be essential for life, for example, for the growth of animals, such as fluorine, silicon, tin, vanadium and nickel.

In many cases the exact role played by these elements would remain unknown or unclear. Both chemists and biologists have long been surprised by the way the evolution has selected certain elements as the building-blocks of living organisms. Thus the composition of the earth and its atmosphere obviously sets a limit on what elements are available.

The solar system, like the universe, seems to be 99% hydrogen and helium. In the earth’s crust helium appears to be essentially non-existent, except in a few rare deposits, hydrogen atoms constituting only 22% of the total.

Eight elements provide more that 98%of the atoms in the earth’s crust, namely oxygen 47%, silicon 28%, aluminium 7.9%, iron 4.5%, calcium 3.5%, sodium 2.5%, potassium 2.5%, magnesium 2.2%. Of these 8 elements only 5 are among the 11 that account for more 99.9 % of the atoms in the human body. Hydrogen and oxygen account for 88.5% of the atoms in the human body, hydrogen supplying 63% of the total and oxygen 25.5%.

Silicon is known to be 146 times more plentiful than carbon in the earth’s crust. Silicon like carbon has the capacity to gain four electrons and form four covalent bonds. Carbon was selected over silicon as the central building-block. The difference that led to the preference for carbon compounds over silicon compounds can be explained: 1) by the unusual stability of carbon dioxide; 2) by almost unique ability of carbon to form long chains and stable rings with five of six members. The versatility of the carbon atom is responsible for the millions of organic compounds found on the earth.

If some generalization were made about the role of various elements it would be interesting to note that six elements carbon, nitrogen, hydrogen, oxygen, phosphorous and sulphur make up the molecular-building blocks of living matter: amino acids, sugars, fatty acids, purines, pyrimidines and nucleotides.

These molecules not only have independent biochemical roles but also are constituents of elements that either gain or lose electrons when they are dissolved in water, thus forming ions.

The principal positively charged ions are provided by four metals: sodium, potassium, calcium and magnesium. The principal anions (ions with negative charge) are provided by the chloride ion, sulphur, and phosphorus. These seven ions maintain the electrical neutrality of a body fluid and cells. This is considered to be the second essential group. The third group of essential elements consists of the trace elements. The fact that they are required in extremely minute quantities in no way diminishes their great importance.