The Carbon Cycle

		Biological/Physical Carbon Cycle: Photosynthesis and Respiration Biology also plays an important role in the movement of carbon in and out of the land and ocean through the processes of photosynthesis and respiration. Nearly all forms of life on Earth depend on the production of sugars from solar energy and carbon dioxide (photosynthesis) and the metabolism (respiration) of those sugars to produce the chemical energy that facilitates growth and reproduction. During photosynthesis, plants absorb carbon dioxide and sunlight to create fuel—glucose and other sugars—for building plant structures. This process forms the foundation of the biological carbon cycle. (Illustration courtesy P.J. Sellers et al.) Through the process of photosynthesis, green plants absorb solar energy and remove carbon dioxide from the atmosphere to produce carbohydrates (sugars). Plants and animals effectively “burn” these carbohydrates (and other products derived from them) through the process of respiration, the reverse of photosynthesis. Respiration releases the energy contained in sugars for use in metabolism and renders the carbohydrate“fuel” back to carbon dioxide. Together, respiration and decomposition (respiration that consumes organic matter mostly by bacteria and fungi) return the biologically fixed carbon back to the atmosphere. The amount of carbon taken up by photosynthesis and released back to the atmosphere by respiration each year is 1,000 times greater than the amount of carbon that moves through the geological cycle on an annual basis. Photosynthesis and respiration also play an important role in the long-term geological cycling of carbon. The presence of land vegetation enhances the weathering of soil, leading to the long-term—but slow—uptake of carbon dioxide from the atmosphere. In the oceans, some of the carbon taken up by phytoplankton (microscopic marine plants that form the basis of the marine food chain) to make shells of calcium carbonate (CaCO3) settles to the bottom (after they die) to form sediments. During times when photosynthesis exceeded respiration, organic matter slowly built up over millions of years to form coal and oil deposits. All of these biologically mediated processes represent a removal of carbon dioxide from the atmosphere and storage of carbon in geologic sediments.