In considering the possibility of life on other worlds we must ask two important questions – outward appearances and biochemical differences. We now consider the biochemical differences.
On Earth life relies on combinations of only a few chemical elements. We refer to it as carbon based life because of the huge variety of chemical structures made possible by the ability of the carbon atoms to form long chains and rings with each other. Reference to The Periodic Table of the Chemical Elements shows that only a few elements play a major part in life processes. TWELVE of the elements seem to be be abundant in living organisms. They are the non-metals carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur and chlorine (as the chloride ion) and the metals sodium, potassium, calcium, magnesium and iron. A strange anomaly is that of the molluscs which includes the octopus and squid use copper compounds instead of iron to transport oxygen in their blood. A number of other elements play a minor role in many life forms namely (cobalt), copper(Cu), zinc(Zn), manganese(Mn), chromium(Cr), vanadium(V), selenium(Se), boron (B), fluorine(F), iodine(I), moybdenum(Mb) and tungsten(W) occur in trace amounts in living organisms and may be essential in tiny quantities to some or even all organisms.
Many science-fiction writers have proposed life-forms that use alternate chemical elements others than those found on Earth – this particularly concerns life forms based on elements other than carbon. Could any chemical element which does not play a major role in the biochemistry of life on Earth play a major role in life elsewhere? A number of elements will now be discussed. Of the alkali metals only sodium and potassium ions are used in life processes. The other alkali metals lithium, rubidium and caesium are much rarer in the Earth and are unlikely to be common in the universe as a whole. This is baring in mind the way in which chemical elements are produced in supernova explosions (See my other website named Visions of the Cosmos). The same arguments apply to the 'group two elements' where magnesium and calcium are unlikely to be replaced by strontium or barium.
Boron is able to make long chains like carbon. Also it can form long chain compounds alternating with nitrogen -BNBNBNBN and so on. Most of the boron-nitrogen compounds are fairly unstable. Also they react chemically with water. Suggestions have been made that boron nitrogen compounds dissolved in liquid ammonia could form the basis of viable biochemical systems. Its main drawback as a candidate for forming a basis of life is its comparative rarity – about 10 parts per million. Because of the way in which the atoms of chemical elements are formed from supernova stars it is also likely to be relatively rare on planets in general whereas carbon is likely to be far more common.
One of the favourite elements mentioned in science fiction as a possible substitute for carbon is silicon. Actually silicon as a basis for life is a very unlikely candidate for such a role. True it has four valency 'arms' as does carbon BUT its oxide is quartz which is a crystalline solid with a very high melting point not a gas like carbon dioxide. Silicon does form hydrides similar to carbon. They are all very unstable - for example Silane SiH4 unlike Methane CH4 is very unstable as are all the other silicon analogues.
An element which has been proposed as a substitute for phosphorus is ARSENIC. Arsenic is positioned just below phosphorus in the periodic table, and the two elements can play a similar role in chemical reactions. For example, the arsenate ion, AsO43-, has the same tetrahedral structure and bonding sites as phosphate. It is so similar that it can get inside cells by hijacking phosphate's transport mechanism which is the reason why arsenic is so toxic to most organisms. In December 2010 it was reported in the journal Nature by Felisa Wolfe-Simon and her co-workers with the NASA Astrobiology unit, that they had discovered that a member of the Halomonadaceae family of proteobacteria that can use arsenic in place of phosphorus. The bacteria was living in Menlo Lake, California. There was much talk at the time that life may indeed exist even on our own planet in which arsenic replaces phosphorus in its DNA.
The newly discovered microbe, strain GFAJ-1, is a member of a common group of bacteria, the Gammaproteobacteria. In the laboratory, the researchers successfully grew microbes from the lake on a diet that was very lean on phosphorus, but included generous helpings of arsenic. When researchers removed the phosphorus and replaced it with arsenic the microbes continued to grow. Subsequent analyses indicated that the arsenic was being used to produce the building blocks of new GFAJ-1 cells. There was much talk at the time that life may indeed exist even on our own planet in which arsenic replaces phosphorus in its DNA. However the findings were strongly opposed in another article in Nature in October 2012 suggesting that the bacteria investigated by the NASA team actually preferred phosphorus to arsenic by a factor of 4000 to 1 even though they may be fairly resistant to the poisonous effects of arsenic.
The oxygen family of elements includes sulphur, selenium, tellurium and polonium. Sulphur is already known to be of importance to life on earth. It occurs in three amino acids and therefore all (or nearly all) proteins. It is found as its sulphide H2S and plays a major role in some anaerobic microorganisms. Nevertheless it would be very unlikely to take the place of oxygen gas O2 and water which plays such a major role in life on Earth. Selenium is now known to form an amino acid selenocysteine in which it takes the place of sulphur. It has not been found widely but does occur in a few proteins found in human metabolism. Both Selenium and Tellurium could theoretically occur in some life processes but would be unlikely to replace sulphur in playing a major role. The last element in the series Polonium only occurs in tiny amounts as highly radioactive isotopes.
Somewhere there are mountains
Glistening in the snow
Somewhere there are mountains
That we shall never know
Somewhere there are rivers
Flowing fast and free
Somewhere there are rivers
That we can never see
Somewhere there are oceans
And sun drenched island sands
Forests full of creatures
In vastly distant lands
Somewhere there’s a planet
Beneath an alien star
The people watch our tiny sun
And wonder where we are
One day perhaps we’ll find them
Across the void of space
Perhaps through ways as yet unknown
We’ll meet them face to face
The author of this web site Ray Goodwin holds B.Sc. Degrees from London University in Chemistry, Geology and Physiology and an M.Sc. in Biochemistry. He has spent most of his professional life teaching in Colleges of Technology. On his retirement he has entered the fields of astronomy, astrochemistry, astrobiology and space sciences. He has spent a great deal of his retirement in visiting amateur astronomy societies and in attending European Space Agency Symposia in ESTEC in the Netherlands and other scientific conferences in England and Sweden. He regularly attends the yearly European Astrofest in South Kensington London and other meetings in the UK. He has written scientific articles and given a number of lectures on diverse scientific subjects.
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