The Carbon Cycle

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THE CARBON CYCLE

CONCEPT

If a person were asked to name the element most important to sustaining life, chances are he or she would say oxygen. It is true that many living things depend on oxygen to survive, but, in fact, carbon is even more fundamental to the sustenance of life. Indeed, in a very real sense, carbon is life, since every living thing contains carbon and the term organic refers to certain varieties present in all life-forms. Yet carbon, in the form of such oxides as carbon dioxide as well as carbonates like calcium carbonate, is a vital part of the inorganic realm as well. Hence, the carbon cycle, by which the element is circulated through the biosphere, geosphere, atmosphere, and hydrosphere, is among the most complex of biogeochemical cycles.

HOW IT WORKS

Geochemistry

Chemistry is concerned with the composition, structure, properties, and changes of substances, including elements, compounds, and mixtures. Central to the discipline is the atomic model, or the idea that all matter is composed of atoms, each of which represents one and only one chemical element. An element thus is defined as a substance made up of only one kind of atom, which cannot be broken chemically into other substances. A chemical reaction involves either the bonding of one atom with another or the breaking of chemical bonds between atoms.

Geochemistry brings together geology and chemistry, though as the subdiscipline has matured in the period since the 1940s, its scope has widened to take in aspects of other disciplines and subdisciplines. With its focus on such issues as the recycling of elements between the various sectors of the earth system, especially between living and nonliving things, geochemistry naturally encompasses biology, botany, and a host of earth science subdisciplines, such as hydrology.

BIOGEOCHEMICAL CYCLES.

Among the most significant areas of study within the realm of geochemistry are biogeochemical cycles. These are the changes that a particular element undergoes as it passes back and forth through the various earth systems—particularly between living and nonliving matter. As we shall see, this transition between the worlds of the living and the nonliving is particularly interesting where carbon is concerned.

Along with carbon, five other elements—hydrogen, nitrogen, oxygen, phosphorus, and sulfur—are involved in biogeochemical cycles. With the exception of phosphorus, which plays little part in the atmosphere, these elements move through all four earth systems, including the atmosphere, the biosphere (the sum of all living things as well as formerly living things that have not yet decomposed), the hydrosphere (Earth's water, except for water vapor in the atmosphere), and the geosphere, or the upper part of Earth's continental crust.

Earth systems and biogeochemical cycles are discussed in greater depth within essays devoted to those topics (see Earth Systems and Biogeochemical Cycles). Likewise, the nitrogen cycle is treated separately (see Nitrogen Cycle). The role of hydrogen and oxygen, which chemically bond to form water, is discussed in the context of the hydrosphere (see Hydrologic Cycle).

Elements and Compounds

We have referred to elements and compounds, which are essential to the study of chemistry; now let us examine them briefly before going on to the subject of a specific and very important element, carbon. An element is defined not by outward characteristics, though elements do have definable features by which they are known; rather, the true meaning of the term element is discernible only at the atomic level.

Every atom has a nucleus, which contains protons, or subatomic particles of positive electric charge. The identity of an element is defined by the number of protons in the nucleus: for instance, if an atom has only a single proton, by definition it must be hydrogen. An atom with six protons in the nucleus, on the other hand, is always an atom of carbon. Thus, the elements are listed on the periodic table of elements by atomic number, or the number of protons in the atomic nucleus.

ELECTRONS AND CHEMICAL REACTIONS.

While protons are essential to the definition of an element, they play no role in the bonding between atoms, which usually produces chemical compounds. (The reason for this is qualified by the modifier usually, in that sometimes two atoms of the same element may bond as well.) Chemical bonding involves only the electrons, which are negatively charged subatomic particles that spin around the nucleus. In fact, only certain of these fast-moving particles take part in bonding. These are the valence electrons, which occupy the highest energy levels in the atom.

One might say that valence electrons are at the "outside edge" of the atom, though the model of atomic structure, considered only in the briefest form here, is far more complex than that phrase implies. In any case, elements have characteristic valence electron patterns that affect their reactivity, or their ability to bond. Carbon is structured in such a way that it can form multiple bonds, and this feature plays a significant part in its importance as an element.

When an element reacts with another, they join together, generally in a molecule (we will examine some exceptions), to form a compound. Though the atoms themselves remain intact, and an element can be released from a compound, a compound quite often has properties quite unlike those of the original elements. Carbon and oxygen are essential to sustaining life, but when a single atom of one bonds with a single atom of the other, they form a toxic gas, carbon monoxide. And whereas carbon in its elemental form is a black powder and hydrogen and oxygen are colorless, odorless gases, when bonded in the proper proportions and structure, the three create sugar.

Carbon

The name carbon comes from the Latin word for charcoal, carbo. In fact, charcoal—wood or other plant material that has been heated without enough air present to make it burn—is just one of many well-known substances that contain carbon. Others include coal, petroleum, and other fossil fuels, all of which contain hydrocarbons, or chemical compounds built around strings of carbon and hydrogen atoms. Graphite is pure carbon, and coke, a refined version of coal, is very nearly pure. Not everything made of carbon is black, however: diamonds, too, are pure carbon in another form.

Though carbon makes up only a small portion of the known elemental mass in Earth's crust, waters, and atmosphere—just 0.08%, or 1/1,250 of the whole—it is the fourteenth most abundant element on the planet. In the human body, carbon is second only to oxygen in abundance and accounts for 18% of the body's mass. Present in the inorganic rocks of the ground and in the living creatures above it, carbon is everywhere in the earth system.

CARBON BONDING.

There are two elements noted for their ability to form long strings of atoms and seemingly endless varieties of molecules: one is carbon, and the other is silicon, directly below it on the periodic table. Just as carbon forms a vast array of organic compounds, silicon, found in a huge variety of minerals, is at the center of a large number of inorganic compounds. Yet carbon is capable of forming an even greater number of bonds than silicon. (For more about silicon and the silicates, see the entries Minerals and Economic Geology.)

Carbon is distinguished further by its high value of electronegativity, the relative ability of an atom to attract valence electrons. In addition, with four valence electrons, carbon is ideally suited to finding other elements (or other carbon atoms) with which to form chemical bonds. Normally, an element does not necessarily have the ability to bond with as many other elements as it has valence electrons, but carbon—with its four valence electrons—happens to be tetravalent, or capable of bonding to four other atoms at once. Additionally, carbon can form not just a single bond but also a double bond or even a triple bond with other elements.

ALLOTROPES OF CARBON.

Carbon has several allotropes—different versions of the same element distinguished by their molecular structure. The first of them is graphite, a soft material that most of us regularly encounter in the form of pencil "lead." Graphite is essentially a series of one-atom-thick sheets of carbon bonded together in a hexagonal pattern, but with only very weak attractions between adjacent sheets.

Then there is that most alluring of all carbon allotropes, diamond. Neither diamonds nor graphite, strictly speaking, are formed of molecules. Their arrangement is definite, as with a molecule, but their size is not: they simply form repeating patterns that seem to stretch on forever. Whereas graphite is in the form of sheets, a diamond is basically a huge "molecule" composed of carbon atoms strung together by what are known as covalent chemical bonds.

Graphite and diamond are both crystalline—solids in which the constituent parts have a simple and definite geometric arrangement that is repeated in all directions. (All minerals are crystalline in structure. See Minerals.) A third carbon allotrope, buckminsterfullerene, discovered in 1985 and named after the American engineer and philosopher R. Buckminster Fuller (1895-1983), is also crystalline in form.

Carbon takes yet another form, distinguished from the other three allotropes in that it is amorphous in structure—lacking a definite shape—as opposed to crystalline. Though it retains some of the microscopic structures of the plant cells in the wood from which it is made, charcoal is mostly amorphous carbon. Coal and coke are particularly significant varieties of amorphous carbon. Formed by the decay of fossils, coal was the first important fossil fuel (discussed later in this essay) used to provide heat and power to human societies.

REAL-LIFE APPLICATIONS

Organic Chemistry

Organic chemistry is the study of carbon, its compounds (with the exception of the carbonates and oxides mentioned earlier), and their properties. At one time chemists thought that organic was synonymous with living, and even as recently as the early nineteenth century, they believed that organic substances contained a supernatural "life force." Then, in 1828, the German chemist Friedrich Wöhler (1800-1882) made an amazing discovery.

By heating a sample of ammonium cyanate, a material from a nonliving source, Wöhler converted it to urea, a waste product in the urine of mammals. As he later observed, "without benefit of a kidney, a bladder, or a dog," he had turned an inorganic substance into an organic one. It was almost as though he had created life. Actually, what he had discovered was the distinction between organic and inorganic material, which results from the way in which the carbon chains are arranged.

Organic chemistry encompasses the study of many things that people commonly think of as "organic"—living creatures, formerly living creatures, and the parts and products of their bodies—but it also is concerned with substances that seem quite far removed from the living world. Among these substances are rubber, vitamins, cloth, and paper, but even in these cases, it is easy to see the relationship to a formerly living organism: a rubber plant, or a tree that was cut down to make wood pulp. But it might come as a surprise to learn that plastics, which at first glance would seem completely divorced from the living world, also have an organic basis. All manner of artificial substances, such as nylon and polyester, are made from hydrocarbons.

FOSSIL FUELS.

During the Mesozoic era, which began about 248.2 million years ago, dinosaurs ruled the earth; then, about 65 million years ago, a violent event brought an end to their world. The cause of this mass extinction is unknown, though it is likely that a meteorite hit the planet, sending so much dust into the atmosphere that it dramatically changed local climates, bringing about the destruction of the dinosaurs—along with a huge array of other animal and plant forms. (See Paleontology for more on this subject.)

The bodies of the dinosaurs, along with those of other organisms, were deposited in the solid earth and covered by sediment. They might well have simply rotted, and indeed many of them probably did. But many of these organisms were deposited in an anaerobic, or non-oxygen-containing, environment. Rather than simply rotting, this organic material underwent transformation into hydrocarbons and became the basis for the fossil fuels, the most important of which—from the standpoint of modern society—is petroleum. (See Economic Geology for more on this subject.)

Carbonates

Carbonates are important forms of inorganic carbon in the geosphere. In chemical terms, a carbonate is made from a single carbon atom bonded to three oxygen atoms, but in mineralogical terms, carbonates are a class of mineral that may contain carbon, nitrogen, or boron in a characteristic molecular formation. Typically, a carbonate is transparent and light in color with a relatively high density. Among carbonate minerals, the most significant compound is calcium carbonate (CaCO₃). One of the most common compounds in the entire geosphere, constituting 7% of the known crustal mass, it is found in such rocks as limestone, marble, and chalk. (Just as pencil "lead" is not really lead, the "chalk" used for writing on blackboards is actually gypsum, a form of calcium sulfate.) Additionally, calcium carbonate can combine with magnesium to form dolomite, and in caves it is the material that makes up stalactites and stalagmites. Yet calcium carbonate also is found in coral, seashells, eggshells, and pearls. This is a good example of how a substance can cross the chemical boundary between the worlds of the living and nonliving.

In the oceans, calcium reacts with dissolved carbon dioxide, forming calcium carbonate and sinking to the bottom. Millions of years ago, when oceans covered much of the planet, sea creatures absorbed calcium and carbon dioxide from the water, which reacted to form calcium carbonate that went into their shells and skeletons. After they died, their bodies became sedimented in the ocean floor, forming vast deposits of limestone.

Carbon Dioxide and Carbon Monoxide

Historically, carbon dioxide was the first gas to be distinguished from ordinary air, when in 1630 the Flemish chemist and physicist Jan Baptista van Helmont (1577?-1644) discovered that air was not a single element, as had been thought up to that time. The name perhaps most closely associated with carbon dioxide, however, is that of the English chemist Joseph Priestley (1733-1804), who created carbonated water, used today in making soft drinks. Not only does the gas add bubbles to drinks, it also acts as a preservative.

By Priestley's era, chemists had begun to glimpse a relationship between plant life and carbon dioxide. Up until that time, it had been believed that plants purify the air by day and poison it at night. Today we know that carbon dioxide is an essential component in the natural balance between plant and animal life. Animals, including humans, breathe in air, and, as a result of a chemical reaction in their bodies, the oxygen molecules (O₂) bond with carbon to produce carbon dioxide. Plants "breathe" in this carbon dioxide (which is as important to their survival as air is to animals), and a reverse reaction leads to the release of oxygen from the plants back into the atmosphere.

CARBON MONOXIDE.

Priestley discovered another carbon-oxygen compound quite different from carbon dioxide: carbon monoxide. The latter is used today by industry for several purposes, such as the production of certain fuels, proving that this toxic gas can be quite beneficial when used in a controlled environment. Nonetheless, carbon monoxide produced in an uncontrolled environment—generated by the burning of petroleum in automobiles as well as by the combustion of wood, coal, and other carbon-containing fuels—is extremely hazardous to human health.

When humans ingest carbon monoxide, it bonds with iron in hemoglobin, the substance in red blood cells that transports oxygen throughout the body. In effect, carbon monoxide fools the body into thinking that it is receiving oxygenated hemoglobin, or oxyhemoglobin. Upon reaching the cells, carbon monoxide has much less tendency than oxygen to break down, and therefore it continues to circulate throughout the body. Low concentrations can cause nausea, vomiting, and other effects, while prolonged exposure to high concentrations can result in death.

THE GREENHOUSE EFFECT.

Although we have referred to carbon monoxide as toxic, it should be noted that carbon dioxide also would be toxic to a human or other animal—for instance, if one were trapped in a sealed compartment and forced to breathe in the carbon dioxide released from one's lungs. On a global scale, both carbon dioxide and carbon monoxide in the atmosphere, produced in excessive amounts by the burning of fossil fuels, pose a potentially serious threat.

Both gases are believed to contribute to the greenhouse effect, which, as discussed in Energy and Earth, is a mechanism by which the planet efficiently uses the heat it receives from the Sun. Human consumption of fossil fuels and use of other products, including chlorofluorocarbons in aerosol cans, however, has produced a much greater quantity of greenhouse gases than the atmosphere needs to maintain normal heat levels. As a result, some scientists believe, buildup of greenhouse gases in the atmosphere is causing global warming.

Cellular Respiration

The burning of fossil fuels is one of three ways that carbon enters the atmosphere, the others being volcanic eruption and cellular respiration. When cellular respiration takes place in the presence of oxygen, there is an intake of organic substances, which are broken down into carbon dioxide and water, with the release of considerable energy.

When plants take in carbon dioxide from the atmosphere, they combine it with water and manufacture organic compounds, using energy they have trapped from sunlight by means of photosynthesis—the conversion of light to chemical energy through biological means. As a byproduct of photosynthesis, plants release oxygen into the atmosphere, as we have noted earlier.

In the process of photosynthesis, plants produce carbohydrates, which are various compounds of carbon, hydrogen, and oxygen that are essential to life. (The other two fundamental components of a diet are fats and proteins, both of which are carbon-based as well.) Animals eat the plants or eat other animals that eat the plants and thus incorporate the fats, proteins, and sugars (a form of carbohydrate) from the plants into their bodies. In cellular respiration, these nutrients are broken down to create carbon dioxide.

DECOMPOSITION.

Cellular respiration also releases carbon into the atmosphere through the action of decomposers, organisms that obtain their energy from the chemical breakdown of dead organisms as well as from animal and plant waste products. Bacteria and fungi, the principal forms of decomposer, extract energy contained in the chemical bonds of the organic matter they are decomposing and, in the process, release carbon dioxide.

Certain ecosystems, or communities of interdependent organisms, are better than others at producing carbon dioxide through decomposition. As one would expect, environments where heat and moisture are greatest—for example, a tropical rainforest—yield the fastest rates of decomposition. On the other hand, decomposition proceeds much more slowly in dry, cold climates such as that of a subarctic tundra.

WHERE TO LEARN MORE

Blashfield, Jean F. Carbon. Austin, TX: Raintree Steck-Vaughn, 1999.

Carbon Cycle: Exploring the Environment (Web site). <http://www.cotf.edu/ete/modules/carbon/efcarbon.html>.

Carbon Cycle Greenhouse Gases Group (CCGG) (Web site). <http://www.cmdl.noaa.gov/ccgg/>.

Chemical Carousel: A Trip Around the Carbon Cycle (Web site). <http://library.thinkquest.org/11226/>.

Frostburg State University Chemistry Helper (Web site). <http://www.chemhelper.com/>.

"Global Carbon Cycle." The Woods Hole Research Center (Web site). <http://www.whrc.org/science/carbon/carbon.htm>.

Knapp, Brian J. Carbon Chemistry. Illus. David Woodroffe. Danbury, CT: Grolier Educational, 1998.

Loudon, G. Marc. Organic Chemistry. Menlo Park, CA: Benjamin/Cummings, 1988.

Sparrow, Giles. Carbon. New York: Benchmark Books, 1999.

Stille, Darlene. The Respiratory System. New York: Children's Press, 1997.

KEY TERMS

AMORPHOUS:

A term for a type of solid that lacks a definite shape. Compare with crystalline.

ATOMIC NUMBER:

The number of protons in the nucleus of an atom. Since this number is different for each element, elements are listed on the periodic table in order of atomic number.

BIOGEOCHEMICAL CYCLES:

The changes that particular elements undergo as they pass back and forth through the various earth systems and particularly between living and nonliving matter. The elements involved in biogeochemical cycles are hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur.

CELLULAR RESPIRATION:

A process that, when it takes place in the presence of oxygen, involves the intake of organic substances, which are broken down into carbon dioxide and water, with the release of considerable energy.

CHEMICAL BONDING:

The joining through electromagnetic force of atoms that sometimes, but not always, represent more than one chemical element. The result is the formation of a molecule.

COMPOUND:

A substance made up of atoms of more than one element, chemically bonded to one another.

CRYSTALLINE SOLID:

A type of solid in which the constituent parts have a simple and definite geometric arrangement that is repeated in all directions.

DECOMPOSERS:

Organisms that obtain their energy from the chemical breakdown of dead organisms as well as from animal and plant waste products. The principal forms of decomposer are bacteria and fungi.

DECOMPOSITION REACTION:

A chemical reaction in which a compound is broken down into simpler compounds, or into its constituent elements. In the earthsystem, this often is achieved through the help of detritivores and decomposers.

ECOSYSTEM:

A term referring to a community of interdependent organisms along with the inorganic components of their environment.

ELECTRON:

A negatively charged particle in an atom, which spins around the nucleus.

ELEMENT:

A substance made up of only one kind of atom. Unlike compounds, elements cannot be broken chemically into other substances.

FOSSIL FUELS:

Fuel derived from deposits of organic material that have experienced decomposition and chemical alteration under conditions of high pressure. These nonrenewable forms of bioenergy include petroleum, coal, peat, natural gas, and their derivatives.

GEOCHEMISTRY:

A branch of the earth sciences, combining aspects of geology and chemistry, that is concerned with the chemical properties and processes of Earth—in particular, the abundance and interaction of chemical elements and their isotopes.

HYDROCARBON:

Any organic chemical compound whose molecules are madeup of nothing but carbon and hydrogenatoms.

ORGANIC:

At one time chemists used the term organic only in reference to living things. Now the word is applied to most compounds containing carbon and hydrogen, thus excluding carbonates (which are minerals) and oxides such as carbon dioxide.

PERIODIC TABLE OF ELEMENTS:

A chart that shows the elements arranged in order of atomic number along with the chemical symbol and the average atomic mass for each particular element.