Carbon Cycle

The carbon cycle is the process in which carbon atoms are recycled over and over again on Earth. Carbon recycling takes place within Earth's biosphere (the region of the Earth that supports life) and between living things and the nonliving environment. Since a continual supply of carbon is essential for all living organisms, the carbon cycle is the name given to the different processes that move carbon from one organism to another. The complete cycle is made up of "sources" that put carbon back into the environment and "sinks" that absorb and store carbon.

Earth's biosphere can be thought of as a sealed container into which nothing new is ever added except the energy from the sun. Since new matter can never be created, it is essential that living things be able to reuse the existing matter again and again. For the world to work as it does, everything has to be constantly recycled. The carbon cycle is just one of several recycling processes, but it may be the most important process since carbon is known to be a basic building block of life. Carbon is the basis of carbohydrates, proteins, lipids, and nucleic acids—all of which form the basis of life on Earth.

Since all living things contain the element carbon, it is one of the most abundant elements on Earth. The total amount of carbon, whether we are able to measure it accurately or not, always remains the same, although carbon regularly changes its form. A particular carbon atom located in someone's eyelash may have at one time been part of some now-extinct species, like a dinosaur. Since the dinosaur died and decomposed millions of years ago, its carbon atoms have seen many forms before ending up as part of a human being. It may have been part of several plants and trees, been free-floating in the air as carbon dioxide, been locked away in the shell of a sea creature and then buried at the ocean bottom, or may have been part of a volcanic eruption. Carbon is found in great quantities in Earth's crust, its surface waters, the atmosphere, and the mass of green plants. It also is found in many different chemical combinations, including carbon dioxide (CO₂), calcium carbonate (CaCO₃), as well as in a huge variety of organic compounds such as hydrocarbons (like coal, petroleum, and natural gas).

CARBON CYCLE PROCESSES

If a diagram were drawn showing the different processes that move carbon from one form to another, its main processes would be photosynthesis, respiration, decomposition, combustion of fossil fuels, and the natural weathering of rocks.

PHOTOSYNTHESIS

Carbon exists in the atmosphere as the compound carbon dioxide. It first enters the ecological food web (the connected network of producers and consumers) when photosynthetic organisms, such as plants and certain algae, absorb carbon dioxide through tiny pores in their leaves. The plants then "fix" or capture the carbon dioxide and are able to convert it into simple sugars like glucose through the biochemical process known as photosynthesis. Plants store and use this sugar to grow and reproduce. When plants are eaten by animals, their carbon is passed on to those animals. Since animals cannot make their own food, they must get their carbon by eating plants or by eating animals that have eaten plants.

RESPIRATION

Respiration is the next step in the cycle, and unlike photosynthesis, it occurs in plants, animals, and decomposers. Although we usually think of breathing oxygen when we hear the word "respiration," it has a broader meaning that involves oxygen. To a biologist, respiration is the process in which oxygen is used to break down organic compounds into carbon dioxide (CO₂) and water (H₂O). For an animal, respiration includes taking in oxygen (and releasing carbon dioxide) and oxidizing its food (or burning it with oxygen) in order to release the energy the food contains. In both cases, carbon is returned to the atmosphere as carbon dioxide. Carbon atoms that started out as components of carbon dioxide molecules have passed through the body of living organisms and been returned to the atmosphere, ready to be recycled again.

DECOMPOSITION

Decomposition is the largest source through which carbon is returned to the atmosphere as carbon dioxide. Decomposers are microorganisms that live mostly in the soil but also in water, and which feed on the rotting remains of plants and animals. It is their job to consume both waste products and dead matter, during which they return carbon dioxide to the atmosphere by respiration. Decomposers not only play a key role in the carbon cycle, but break down, remove, and recycle what might be called nature's garbage.

HUMANS INCREASE CARBON DIOXIDE IN THE ATMOSPHERE

In recent history, humans have added to the carbon cycle by burning fossil fuels. Ever since the rapid growth of the Industrial Revolution when people first harnessed steam to power their engines, human beings have been burning carbon-containing fuels like coal and oil for artificial power. This constant burning produces massive amounts of carbon dioxide, which are released into Earth's atmosphere. Fossil fuel consumption could be an example of a human activity that affects and possibly alters the natural processes (photosynthesis, respiration, decomposition) that nature had previously kept in balance. Many scientists believe that carbon dioxide is a "greenhouse gas." This means that it traps heat and prevents it from escaping from Earth. As a result, this trapped gas leads to a global temperature rise, which can have disastrous effects on Earth's environment.

Not all carbon atoms are always moving somewhere in the carbon cycle. Often, many become trapped in limerock, a type of stone formed on the ocean floor by the shells of marine plankton. Sometimes after millions of years, the waters recede and the limerock is eventually exposed to the elements. When limerock is exposed to the natural process of weathering, it slowly releases the carbon atoms it contains, and once again they become an active part of the carbon cycle.

[See alsoBiosphere; Carbon Dioxide; Decomposition; Photosynthesis; Respiration ]