Carbon Cycle

Plants convert the carbon in atmospheric carbon dioxide into carbon-containing organic compounds, such as sugars, fats, and proteins. Plants take in carbon dioxide through microscopic openings in their leaves, called stomata. They combine atmospheric carbon with water and manufacture organic compounds, using energy trapped from sunlight in a process called photosynthesis. The by-product of photosynthesis is oxygen, which plants release into the atmosphere through the stomata.

Animals that eat plants, or that eat other animals, incorporate the carbon in the sugars, fats, and proteins derived from the ingested biomass into their bodies. Inside their cells, energy is extracted from the food in a process called cellular respiration. Cellular respiration requires oxygen (which is the by-product of photosynthesis) and it produces carbon dioxide, which is used in photosynthesis. In this way, photosynthesis and cellular respiration are linked in the carbon cycle.

Photosynthesis requires atmospheric carbon, while cellular respiration returns carbon to the atmosphere, and vice versa for oxygen. The global rates of photosynthesis and cellular respiration influence the amount of carbon dioxide in the atmosphere. In the summer, the high rate of photosynthesis uses up much of the carbon dioxide in the atmosphere, and the amount of atmospheric carbon dioxide decreases. In the winter, when the rate of photosynthesis is low, the amount of atmospheric carbon dioxide increases.

Another way that cellular respiration releases carbon into the atmosphere is through the actions of decomposers. Decomposers, such as bacteria and fungi, derive their nutrients by feeding on the remains of plants and animals. The bacteria and fungi use cellular respiration to extract the energy contained in the chemical bonds of the decomposing organic matter, and so release carbon dioxide into the atmosphere.

In some ecosystems, such as tropical rainforests, decomposition is accomplished quickly, and carbon dioxide is returned to the atmosphere at a relatively fast rate. In other ecosystems, such as northern forests and tundra, decomposition proceeds more slowly. In some places, such as bogs and the deep ocean, the organic matter of plants and animals may accumulate in deep sediments, where decomposers cannot function well because of the lack of oxygen. Slowly, over millions of years, the carbon-rich materials are converted into carbon-rich fossil fuels, such as petroleum, natural gas, and coal. Also in marine environments, carbon-containing matter (such as calcium carbonate) is incorporated into the shells and other hard parts of aquatic organisms. When these organisms die, the carbon-rich hard parts sink to the ocean bed. There they become buried in sediment, and eventually densify into rocks such as limestone and dolomite.