The Carbon Cycle

Photosynthesis and cellular respiration form the basis of the short-term carbon cycle, illustrated in Figure 18-13. In photosynthesis, plants and other autotrophs use carbon dioxide (CO2), along with water and solar energy, to make carbohydrates. Both autotrophs and heterotrophs use oxygen to break down carbohydrates during cellular respiration. The byproducts of cellular respiration are carbon dioxide and water. Decomposers release carbon dioxide into the atmosphere when they break down organic compounds.

Carbon dioxide in atmosphere

Carbon dioxide in atmosphere

Human Influences on the Carbon Cycle

In the last 150 years, the concentration of atmospheric carbon dioxide has risen more than 30 percent. Humans contribute to this increase by burning fossil fuels and other organic matter. Our industrial society depends on the energy released by the burning of fossil fuels—coal, oil, and natural gas. Fossil fuels are the remains of organisms that have been transformed by decay, heat, and pressure into energy-rich molecules. Burning releases the energy in these molecules, but it also releases carbon dioxide. When large areas of forest are burned each year to clear land for agriculture, less vegetation remains to absorb carbon dioxide from the atmosphere through photosynthesis.

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Maintained by the f 11 ijLs'i* National Science r i A, J, Teachers Association

Maintained by the f 11 ijLs'i* National Science r i A, J, Teachers Association

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