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Ecosystems

Author(s): Deanne Erdmann MS

Carbon Cycle

Carbon, in the form of carbon dioxide, comprises about 0.03 percent of the atmosphere. Worldwide circulation of carbon atoms is called the carbon cycle. Since carbon becomes incorporated into molecules used by living organisms during photosynthesis, parts of the carbon cycle closely parallel the flow of energy through the Earth’s living systems. Carbon is found in the atmosphere, the oceans, soil, fossil deposits and living organisms. Photosynthetic organisms create carbon-containing molecules (known as “organic” compounds), which are passed to other organisms as depicted in food webs. Each year, about 75 billion metric tons of carbon are trapped in carbon-containing compounds through photosynthesis. Carbon is returned to the environment through respiration (breakdown of sugar or other organic compounds), combustion (burning of organic materials, including fossil fuels), and erosion.