How many of the 118 known elements in the periodic table could we permanently delete without any ill effect on our lives? (continued)
Guy Cox
Sydney, Australia
Previous correspondent Mike Follows is far too stingy in his list of elements essential for life. The mnemonic 鈥� carbon (C), hydrogen (H), oxygen (O), phosphorus (P), potassium (K), iodine (I), nitrogen (N), sulphur (S), calcium (Ca), iron (Fe) and magnesium (Mg) 鈥� teaches us the essential elements for plant growth, to which we also have to add sodium and chloride (no salt on the table in this cafe?). Magnesium, by the way, is required for chlorophyll, which enables photosynthesis to take place.
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But there are essential trace elements we humans also need in small quantities 鈥� copper, cobalt, selenium, manganese, molybdenum, zinc. Now we are up to 19 elements.
Silicon isn鈥檛 just useful for computer chips, it is vital for many organisms, most obviously diatoms 鈥� aquatic algae with silica skeletons 鈥� and sponges. Getting somewhat left-field, some ascidians (sea squirts) need vanadium.
Correspondent Eric Kvaalen mentions that polonium is useful for killing people and there is one famous case of that (the 2006 ).
Silicon isn鈥檛 just useful for computer chips, it is vital for many organisms, most obviously diatoms and sponges
But that isn鈥檛 the only use of polonium. Back when photography wasn鈥檛 digital, one of my prized possessions was an anti-static brush that had a strip of polonium behind the hairs to neutralise the static created by brushing when cleaning film. They probably went off the market after that assassination!
Colin Nicholson
Stockport, Greater Manchester, UK
It is indeed interesting to consider which elements we can do without, but we have to be careful. I find it fascinating that evolution has come up with very different strategies in various phyla for the transportation of oxygen and carbon dioxide in the blood and the transfer of energy.
As mammals, we depend on iron atoms at the centre of haemoglobin molecules in our blood, as do fish, from which we may have evolved.
In crustaceans, octopus, spiders and scorpions, the oxygen-absorbing molecule hemocyanin is based on copper, while plants use magnesium in their chlorophyll to capture energy from sunlight.
Insects don鈥檛 have the veinous system of us animals, but absorb oxygen directly through tubules in their skin. Their bodies are filled with a hemolymph based on large protein molecules containing copper, but this has evolved for other uses, not oxygen transport.
Howard Homler
Sacramento, California, US
Correspondent Simon Goodman forgot the iron in haemoglobin, which is essential in much more than trace amounts, only listing it as a structural metal.
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