Monday, March 19, 2012

Carbon Sequence

To catch the attention of high school students beginning their study of the carbon cycle, I may recycle one or two simple demonstrations they might remember from their younger days. One is a favorite, outrageous party trick. Passing one’s finger quickly through the lower part of a candle flame leaves a strip of black soot. The black material turns out to be non-combusted carbon, one of the elements in an unburned candle. Another experiment reinforces the knowledge that all animals, including humans, exhale carbon dioxide, a product of cellular respiration. Exhaling our breath into colorless limewater through a straw, the limewater turns cloudy. Milky calcium carbonate is formed in the clear limewater. Both reactions exemplify rotation of carbon through various compounds by chemical reactions.

The element carbon is essential to all living things; life cannot exist without carbon. It is the basis of all known earth life. Carbon is the fourth most abundant element in our universe and the second most abundant element in the human body. One source claims “The whole of the living world is based upon compounds of carbon.” It ranks at the top of the “big four:” carbon, hydrogen, nitrogen, and oxygen. But that is not to say life could succeed without one of the others, or even one of the many lesser known elements of which living things are composed.

We must briefly digress. The Big Bang was not a destructive explosion. Rather, it was an event of incredible precision, as were events in the minutes, years, and eons to follow. Cosmologists are in essential agreement concerning the universe’s sequence of events since the Big Bang, acknowledged to be the beginning of time. When I taught the Big Bang event to my astronomy students, I announced it as “God’s initial creation event”--the beginning of the time, space, matter, and energy dimensions of our universe. Much later than this “initial creation event,” the element carbon was produced in the formation of stars by what is known as the triple-alpha process. When supernatural creation events such as the Cambrian Explosion and the creation of modern humans occurred, carbon was already present on earth, ready to be incorporated into the bodies of earth’s living creatures.

Fast forward to life in our day: Carbon, the vital element, now follows cyclical pathways into and out of both living things and non-living matter. The process is an occasion for humble wonder and awe. No new carbon atoms are produced. Therefore, the needs of earth’s living things depend on recycling the elements already present on earth. The most familiar aspect of the carbon cycle is manifest in the production of a carbon compound--carbon dioxide--by the process of cellular respiration in animal life. In turn, the plant’s production of food also results in generation of oxygen as a by-product of the plant’s food synthesis. Then the cycle begins anew.

Carbon is also released by the decay of dead animal and plant matter, combustion, expulsion of dissolved carbon dioxide from the ocean, and volcanoes and hydrothermal vents in the ocean. Other less familiar exchanges of carbon occur between earth’s rocks and the atmosphere. New knowledge of carbon cycle operation is still being discovered.

The simplicity of the well-known exchange of carbon and oxygen between animals and plants belies the wondrous complexity of carbon’s unique properties and the myriad of compounds it can form. Its four valence electrons give it almost limitless opportunities to form compounds. It can form ten million known compounds, but the number of possible carbon compounds is virtually without limit. Organic chemistry is the science of carbon based substances. The variety of carbon allotropes (physical forms), carbon compounds, and the properties of each is far beyond the comprehension of any human being.

Carbon atoms in both diamond and graphite, allotropes of carbon, are not combined with atoms of any other element. The two different substances differ only in their crystal structure, but they are polar opposites in several important physical characteristics. Diamond is transparent, extremely hard, not a conductor of electricity, and very expensive in the form of gem quality jewelry. In contrast, graphite is opaque, very soft, conducts electricity, and an inexpensive, common mineral. Other recently developed allotropes of carbon are fullerenes such as buckyballs and carbon nanotubes, somewhat reminiscent of science fiction imagination. This knowledge of carbon’s allotropes could be cited as a reason to claim our Creator has a sense of humor.