Carbon and Life

It is hard to overstate the importance of carbon; its unique capacity for forming multiple bonds and chains at low energies makes life as we know it possible, and justifies an entire major branch of chemistry - organic chemistry - dedicated to its compounds. In fact, most of the compounds known to science are carbon compounds, often called organic compounds because it was in the context of biochemistry that they were first studied in depth.

What makes carbon so special is that every carbon atom is eager to bond with as many as four other atoms. This makes it possible for long chains and rings to be formed out of them, together with other atoms - almost always hydrogen, often oxygen, sometimes nitrogen, sulfur or halides. The study of these is the basis of organic chemistry; the compounds carbon forms with metals are generally considered inorganic. Chains and rings are fundamental to the way carbon-based life forms - that is, all known life-forms - build themselves. Silicon is capable of forming the same sorts of bonds and structures, but opinion is divided on whether silicon-based life forms are a realistic prospect - in part because it needs higher energies to form them, and in part because whereas carbon dioxide (one of the main by-products of respiration, a process essential to all known life) is a gas and therefore easy to remove from the body, its counterpart silicon dioxide (silica) has an inconveniently high melting point, posing a serious waste disposal problem for any would-be silicon-based life form.

Fuel

Almost everything we use as fuel, whether in food or power stations, is also based on one kind of carbon-based chain or another; everything from natural gas through petrol and alcohol to oil to wax and plastic is composed of hydrocarbon chains of various lengths. As a general rule, the longer the chains and the more saturated they are (meaning every atom of carbon bonds to four other atoms), the less volatile and more viscous the substance will be, and the higher its evaporation temparature. This spread of evaporation temperatures makes it possible to separate the wide range of different hydrocarbons present in crude oil through a process known as fractional distillation, heating the oil to a range of temperatures in sequence and collecting the vapors released. The other big class of carbon-based fuel (used universally by plants and animals, but rarely by power stations) is the carbohydrate family - sugar molecules and the things you can make from them, like starch, cellulose and so on.

Elemental Carbon

Besides its millions of compounds, carbon also bonds with itself in different ways to form graphite, diamond, fullerenes and amorphous solids. Under certain conditions, it can also form white carbon - a transparent, biregringent material about which little else seems to be known.

In graphite, probably best known for its use in pencil 'lead' (for which purpose it is mixed with clay), the carbon is loosely bonded in crystalline layers which are able to slip over one another, so it is relatively soft and deposits itself readily on flat surfaces when drawn over them. It is used as a lubricant, in clay, and as an electrical conductor - one of the few solid, non-metallic conductors. Diamond, by contrast, is the hardest substance known to man; it has a dense, extremely stable crystalline structure, and conducts heat extraordinarily well but electricity hardly at all. Thanks to its hardness it has many industrial applications, while its high refractive index gives jewellery a sparkle which is hard to match.

Another form of elemental carbon, the fullerene, was not observed until 1985, although the possibility of its existence had been suggested as long ago as 1966. The most basic fullerene, (C₆₀, the buckyball - short for buckminsterfullerene) has the form of a football - that's a soccer ball if you American, or a truncated icosahedron if you're a mathemtician - a spherical network of hexagons and pentagons. To the middle of this, rings of hexagons can be added to make bigger fullerenes - anywhere from one ring (C₇₀) to thousands, making carbon nanotubes of anything up to a millimetre long. Tubes can also exist without the hemispherical caps at either end, but in which case they're technically not fullerene because they don't include the twelve pentagons necessary to bend a sheet of hexagons into a closed shape.

The discovery of fullerenes was greeted with a flurry of excitement from chemists, and a huge number of possible uses were suggested; many of these (for instance, the idea that it might be a handy lubricant, like tiny ball bearings) turned out to be quite impractical, but it retains promise as a superconductor, a possible drug delivery vector, and a component of scanning tunnelling microscopes - attaching to the metal tip to increase resolution. The tubes, meanwhile, have extraordinary tensile strength and conduct electricity smoothly along their hollow middle, suggesting applications in nanoscale electronics, and in the production of super-strong, light fibres and resilient new materials. Doubtless many ingenious uses for both the balls and the tubes still remain to be dreamt up.

Isotopes

Carbon exists in three naturally-occurring isotopes - carbon-12, carbon-13, and carbon-14; the numbers refer to their atomic weights. The different kinds of carbon have very similar (but not quite identical) chemical and mechanical properties; the main practical significance of the existence of different isotopes stems from the fact that carbon-14 is radioactive, with a half-life of a few thousand years. This is what makes carbon dating possible - C-14 is thought to be created by impacts from cosmic rays at around the same rate it disappears through radioactive decay, so the ratio of the isotopes in the environment should stay more or less constant. Living organisms constantly exchange carbon with their environment until the day they die - so if we find a skull with only half as much C-14 in it as we see in our environment, we can infer that its owner died one half-life of C-14 ago - that's about 5,600 years. The technique is extremely useful in archaeology, but it is hard to know quite how accurate it is - the assumption that C-14 levels in the atmosphere have remained approximately constant is a difficult one to test.

The Carbon Cycle

The carbon cycle is crucial to the way the Earth's ecosystem works: Plants absorb the carbon dioxide that makes up around 0.03% of our atmosphere and use the energy of sunlight to build sugar molecules from water and CO₂, a process known as photosynthesis. The details of this are still not fully understood by scientists, although work continues apace; it is hoped that a fuller understanding of photosynthesis may one day allow us to create more efficient solar cells, among other things. When the plants are eaten by animals, or decomposed by fungi, or burnt, the stored energy and carbon are released back into the environment and the cycle begins again.

The release of the carbon back into the environment may not happen straight away - when a forest is buried by a landslide, for example, trapping the stored carbon beneath the earth. This sort of thing has happened often enough over the course of the Earth's history to build up large reserves of fossil fuel in the Earth's crust (coal, oil and gas), which we are steadily burning our way through to generate power.

Carbon Emissions

Ever since the Industrial Revolution, humankind's ever-growing use of carbon-based fuels has thrown the carbon cycle somewhat out of balance, resulting in a significant, steady growth in the levels of carbon dioxide in our atmosphere - by about 30% since the start of the last century. It is thought by most environmental scientists that this, together with the rise in more potent 'greenhouse gases' like methane and the chlorofluorocarbons, is probably going to lead to global warming, if it hasn't already started to do so. This is because, like a greenhouse, these gases are transparent to visible light, and to many of the other wavelengths present in sunlight, but reflect infrared; so the energy comes in largely unobstructed and heats the earth below, which then radiates it as infrared back to the upper atmosphere, where the CO₂ bounces it back down to Earth again.

There remains some scientific doubt over the reality and importance of the Greenhouse Effect, but regrettably if it is a real problem then we need to act now to stave off its worst effects. These are likely to include the flooding of low-lying lands the world over; the shifting of prime growing belts a couple of hundred miles away from the equator; a dangerous reduction in biodiversity as environmental conditions change quicker than many animals and plants can adapt to them; and the freezing over of Europe as the Gulf Stream changes its course, taking its warming currents elsewhere.

Acknowledging the undesirability of most of these consequences, a hundred or so of the world's countries got together to agree on the Kyoto Protocol, an international mechanism for the reduction of carbon emissions. The future of this agreement (which most environmentalists feel doesn't go far enough to prevent most of the damage) is uncertain as the United States - having argued for and won many concessions in the protocol, mainly in favour of big business, and gone on to sign but not ratify the treaty - has now pulled out entirely, complaining that we still can't really be certain that inaction will lead to environmental catastrophe and that it would cost an awful lot of money to do anything about it, and also that it was unfair because it doesn't make the same demands of the world's poorer countries as it does of them. The world's top carbon-emitting countries are as follows:

(Table taken from a page at  http://450.aers.psu.edu, since removed - originally from Carbon Dioxide Information Analysis Center, Online Trends: A Compendium of Data on Global Change)

The Periodic Table

Each chapter of Primo Levi's classic The Periodic Table is themed around a different chemical element, sometimes loosely, sometimes less so. In the final part he tells the story of a single atom of carbon, with far greater poetry and scientific detail than I could hope to replicate: How it is trapped for millions of years in a changeless bed of limestone, until chipped free with a pickaxe and delivered to a lime kiln, where the heat frees it as a carbon dioxide molecule; how it is blown back and forth on its escape, sucked up by a falcon, exhaled, dissolved and expelled from oceans and torrents, then blown freely on the wind once more until it happens to meet with a vine where it is built into a sugar molecule.

It is then made into wine, drunk, digested, exhaled, photosynthesised again, eaten by a wood-worm, consumed by microbial decomposers, and breathed out again to fly three times around the world before coming to rest at last in 1960. The story is at once utterly arbitrary, and entirely true; carbon atoms pass along these sorts of paths in such unimaginably huge numbers that it simply could not fail to be so.

Sources include the Encyclopaedia Britannica, Eric Weisstein's World of Chemistry, and various pieces on Everything2, where this article also appears (here).

with thanks to Everything2 users esapersona, Siobhan, wrinkly, LadySun, Professor Pi, Tiefling and Impartial for their helpful advice and suggestions.

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