We just learned that some chemical forms of an element are usable by heterotrophic organisms while others are not. An example of this occurs in the cycling between the chemical forms of carbon. Carbon is the basic building block of organic compounds.

During photosynthesis, carbon dioxide is used to generate oxygen and carbohydrates, which are used for chemical energy. The carbon is incorporated back into carbon dioxide during cellular respiration. Photosynthesis and cellular respiration cycle carbon between forms that are unusable and usable by heterotrophs.

Carbon in the atmosphere is found mainly as carbon dioxide. Primary producers, like plants, use carbon dioxide to produce carbohydrates. The plants are eaten by primary consumers, which are eaten by higher level consumers. When plants and animals die, they become food for detritivores. Cellular respiration by plants, animals, and detritivores, and the burning of fuels, return carbon dioxide to the atmosphere, and the cycle begins again.

Carbon dioxide is cycled out of the atmosphere rapidly, because plants need it for photosynthesis. Each year, plants remove about 15% of the carbon dioxide in the atmosphere. This is roughly balanced by the amount of carbon dioxide released from cellular respiration.

The balance between carbon dioxide produced and released isn't perfect, because some carbon is removed from the cycle for a period of time. Some carbon is diverted from the cycle because it's incorporated in wood or other durable organic material. The carbon is eventually returned to the cycle when it is broken down by detritivores or burned. Certain conditions can cause organic material to be buried and isolated before it can be decomposed. These deposits become fossil fuels, which include petroleum and coal. Fossil fuels are composed of the remains of organisms preserved in the Earth's crust. Carbon is also removed from the cycle in the form of calcium carbonate found in the shells of some aquatic organisms. As it accumulates, the calcium carbonate forms limestone.

The amount of carbon dioxide in the atmosphere is also affected by the season. The level of atmospheric carbon dioxide is lowest during summer in the Northern Hemisphere. When we think about the carbon cycle, this makes perfect sense. The Northern Hemisphere has a larger land mass, and more vegetation than the Southern Hemisphere. Therefore, more photosynthesis occurs because of the vegetation in the Northern Hemisphere. And more photosynthesis occurs in the summer than the winter.

One aspect of the carbon cycle we haven't discussed is the role of aquatic carbon. Carbon is found in several chemical forms in water. Atmospheric carbon dioxide dissolves in water. The dissolved carbon dioxide reacts with water to form carbonic acid. The carbonic acid then reacts with calcium carbonate in shells and limestone to form bicarbonate, which reacts to form carbonate.

All of these reactions are in equilibrium, so the amount of one chemical form affects the amount of the other forms. As dissolved carbon dioxide is used by aquatic plants for photosynthesis, all the equilibria shift to the left to produce more carbon dioxide. If the level of atmospheric carbon dioxide increases, more carbon dioxide is dissolved in the water, and all the equilibria shift to the right. This is important because it means the oceans help keep the level of atmospheric carbon dioxide relatively stable.

The role of the oceans in buffering the atmospheric carbon dioxide level is increasingly important because of the involvement of humans. One way humans are affecting the environment is by releasing large amounts of carbon dioxide into the atmosphere through the burning of fossil fuels. Fossil fuels are the petroleum we use to run cars and the coal we burn to operate factories. In the last 2 decades, the burning of fossil fuels by humans has released close to 80 billion metric tons of carbon dioxide into the atmosphere. However, the amount of atmospheric carbon dioxide has only increased by about half that amount, probably because the oceans have absorbed most of the difference.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education