The Carbon Cycle (C Cycle)

Carbon is the basic building block of the carbohydrates, proteins, fats, DNA, and other organic compounds that are necessary for life; so organisms must have carbon available to them. Carbon makes up approximately 0.037% of the atmosphere as carbon dioxide gas.

The carbon cycle is the biogeochemical cycle by which carbon is exchanged between the biosphere, geosphere, hydrosphere, and atmosphere of the earth (Fig. 1). The carbon cycle is central to understanding issues related to climate change.

Carbon exists in the Earth's atmosphere primarily as the gas carbon dioxide (CO₂). Although it comprises a very small part of the atmosphere overall (approximately 0.04 percent), it plays an important role in supporting life. Other gases containing carbon in the atmosphere are methane and chlorofluorocarbons.

The carbon cycle is based on carbon dioxide gas, which is a key component of nature’s thermostat. If the carbon cycle removes too much carbon dioxide from the atmosphere, the atmosphere will cool; if the cycle generates too much, the atmosphere will get warmer. Thus even slight changes in the carbon cycle can affect climate and ultimately the types of life that can exist on various parts of the planet.

                                         Fig. 1: The Carbon Cycle

During photosynthesis plants, algae, and certain bacteria remove carbon dioxide from the air and fix, or incorporate, it into complex chemical compounds such as sugar. Plants use sugar to make other compounds. Those compounds are usually used as fuel for cell respiration by the producer that made them, by a consumer that eats the producer, or by a decomposer that breaks down the remains of the producer or consumer. This linkage between photosynthesis and cellular respiration is a major part of the global carbon cycle

A large amount of carbon is stored in the wood of trees, coal, oil, and natural gas, called fossil fuels. Burning or combustion of the fossil fuels releases carbon dioxide to the atmosphere, which then is ready to enter the C cycle again.

An even greater amount of carbon is incorporated into the shells of marine organisms. When these organisms die, their shells sink to the ocean floor and are covered by sediments, forming seabed deposits thousands of feet thick. The deposits are eventually cemented together to form a sedimentary rock called limestone. After limestone is exposed (by the process of geologic uplift), it slowly erodes away by chemical and physical weathering processes. This returns the carbon to the water and atmosphere, where it is available to participate in the carbon cycle once again.

Thus, photosynthesis removes carbon dioxide from the abiotic environment and incorporates it into biological molecules, and cell respiration, combustion, and erosion return the carbon in biological molecules to the water and atmosphere of the abiotic environment.

An important aspect of C cycle is that it is the cycle by which solar energy is transferred to biological systems Microorganisms are strongly involved mediating crucial biochemical reactions.

Human Influence on Carbon Cycle

Human activities are increasingly disturbing the balance of the carbon cycle. First, we clear trees and other plants that absorb carbon dioxide through photosynthesis.

 Second, we add large amounts of carbon dioxide by burning fossil fuels and wood.

Increased concentrations of carbon dioxide and other gases we are adding to the atmosphere could enhance the planet’s natural greenhouse effect. The resulting global warming could disrupt global food production and wildlife habitats, alter temperatures and precipitation patterns, and raise the average sea level in various parts of the world. It could force the displacement of thousands or millions of people.