Carbon, an element discovered before history itself, is one of the most abundant elements in the universe. It can be found in the sun, the stars, comets, and the atmospheres of most planets. There are close to ten million known carbon compounds, many thousands of which are vital to the basis of life itself (WWW 1). Carbon occurs in many forms in nature. One of its purest forms is diamond. Diamond is the hardest substance known on earth. Although diamonds found in nature are colorless and transparent, when combined with other elements its color can range from pastels to black. Diamond is a poor conductor of heat and electricity. Until 1955 the only sources of diamond were found in deposits of volcanic origin. Since then scientists have found ways to make diamond from graphite and other synthetic materials. Diamonds of true gem quality are not made in this way (Beggott 3-4).
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Graphite is another form of carbon. It occurs as a mineral in nature, but it can be made artificially from amorphous carbon. One of the main uses for graphite is for its lubricating qualities. Another is for the “lead” in pencils. Graphite is used as a heat resistant material and an electricity conductor. It is also used in nuclear reactors as a lubricator (Kinoshita 119-127). Amorphous carbon is a deep black powder that occurs in nature as a component of coal. It may be obtained artificially from almost any organic substance by heating the substance to very high temperatures without air. Using this method, coke is produced from coal, and charcoal is produced from wood. Amorphous carbon is the most reactive form of carbon. Because amorphous carbon burns easily in air, it is used as a combustion fuel. The most important uses for amorphous carbon are as filler for rubber and as a black pigment in paint (WWW 2). There are two kinds of carbon compounds. The first is inorganic. Inorganic compounds are binary compounds of carbon with metals or metal carbides. They have properties ranging from reactive and saltlike; found in metals such as sodium, magnesium, and aluminum, to an unreactive and metallic, such as titanium and niobium (Beggott 4). Carbon compounds containing nonmetals are usually gases or liquids with low boiling points. Carbon monoxide, a gas, is odorless, colorless, and tasteless. It forms during the incomplete combustion of carbon (Kinoshita 215-223). It is highly toxic to animals because it inhibits the transport of oxygen in the blood by hemoglobin (WWW 2).
Carbon dioxide is a colorless, almost odorless gas that is formed by the combustion of carbon. It is a product that results from respiration in most living organisms and is used by plants as a source of carbon. Frozen carbon dioxide, known as dry ice, is used as a refrigerant. Fluorocarbons, such as Freon, are used as refrigerants (Kinoshita 225-226). Organic compounds are those compounds that occur in nature. The simplest organic compounds consist of only carbon and hydrogen, the hydrocarbons. The state of matter for organic compounds depends on how many carbons are contained in it. If a compound has up to four carbons it is a gas, if it has up to 20 carbons it is a liquid, and if it has more than 20 carbons it is a solid (Kinoshita 230-237). The carbon cycle is the system of biological and chemical processes that make carbon available to living things for use in tissue building and energy release (Kinoshita 242). All living cells are composed of proteins consisting of carbon, hydrogen, oxygen, and nitrogen in various combinations, and each living organism puts these elements together according to its own genetic code. To do this the organism must have these available in special compounds built around carbon. These special compounds are produced only by plants, by the process of photosynthesis.
Photosynthesis is a process in which chlorophyll traps and uses energy from the sun in the form of light. Six molecules of carbon dioxide combine with six molecules of water to form one molecule of glucose (sugar). The glucose molecule consists of six atoms of carbon, twelve of hydrogen, and six of oxygen. Six oxygen molecules, consisting of two oxygen atoms each, are also produced and are discharged into the atmosphere unless the plant needs energy to live. In that case, the oxygen combines with the glucose immediately, releasing six molecules of carbon dioxide and six of water for each molecule of glucose (Beggott 25-32). The carbon cycle is then completed as the plant obtains the energy that was stored by the glucose. The length of time required to complete the cycle varies. In plants without an immediate need for energy, the chemical processes continue in a variety of ways. By reducing the hydrogen and oxygen content of most of the sugar molecules by one water molecule and combining them to form large molecules, plants produce substances such as starch, inulin , and fats and store them for future use. Regardless of whether the stored food is used later by the plant or consumed by some other organism, the molecules will ultimately be digested and oxidized, and carbon dioxide and water will be discharged. Other molecules of sugar undergo a series of chemical changes and are finally combined with nitrogen compounds to form protein substances, which are then used to build tissues (WWW 2). Although protein substances may pass from organism to organism, eventually these too are oxidized and form carbon dioxide and water as cells wear out and are broken down, or as the organisms die. In either case, a new set of organisms, ranging from fungi to the large scavengers, use the waste products or tissues for food, digesting and oxidizing the substances for energy release (WWW 1).
At various times in the Earth’s history, some plant and animal tissues have been protected by erosion and sedimentation from the natural agents of decomposition and converted into substances such as peat, lignite, petroleum, and coal. The carbon cycle, temporarily interrupted in this manner, is completed as fuels are burned, and carbon dioxide and water are again added to the atmosphere for reuse by living things, and the solar energy stored by photosynthesis ages ago is released (Kinoshita 273-275). Almost everything around us today has some connection with carbon or a carbon compound. Carbon is in every living organism. Without carbon life would not exist as we know it.
1. Beggott, Jim Great Balls of Carbon New Scientist, July 6, 1991
2. Kinoshita, Kim Carbon Compounds Random, New York 119-275 1987
3. WWW Carbon http://www.usc.edu/chem/carbon.html 1995
4. WWW Carbon Compounds http://www.harvard.edu/depts/chem/carbon.html 1995