Which statement best describes the carbon cycle?

Which statement best describes the carbon cycle?

• A. The movement of carbon through rocks
• B. The exchange of carbon between plants and soils
• C. The series of processes by which carbon compounds are interconverted in the environment, chiefly involving the incorporation of carbon dioxide into living tissue by photosynthesis and its return to the atmosphere through respiration, the decay of dead organisms, and the burning of fossil fuels
• D. The cycling of carbon solely in aquatic systems

Answer: (C)

The idea being tested is how carbon moves and changes form across the whole Earth system in a connected cycle. The best description shows that carbon compounds are continually interconverted and exchanged among the atmosphere, living things, soils, oceans, and rocks. It starts with carbon dioxide being taken from the air by plants to make organic matter through photosynthesis. That carbon is then returned to the air when organisms respire, when dead matter decays, or when soils release carbon. Carbon can be stored for long periods in soils and rocks as fossil fuels or other reservoirs, and those stores release carbon back to the atmosphere when fossil fuels are burned or weathering eventually brings them to the surface. The oceans also play a crucial role, exchanging CO2 with the atmosphere and hosting carbon in dissolved form and within marine organisms. This interconnected web explains carbon’s continual cycling on short and long timescales. Other descriptions are narrower: focusing only on rock movement ignores the biological and atmospheric exchanges; plant–soil exchange excludes other reservoirs; and aquatic-only cycling misses land, atmosphere, and geological aspects integral to the cycle.

The idea being tested is how carbon moves and changes form across the whole Earth system in a connected cycle. The best description shows that carbon compounds are continually interconverted and exchanged among the atmosphere, living things, soils, oceans, and rocks. It starts with carbon dioxide being taken from the air by plants to make organic matter through photosynthesis. That carbon is then returned to the air when organisms respire, when dead matter decays, or when soils release carbon. Carbon can be stored for long periods in soils and rocks as fossil fuels or other reservoirs, and those stores release carbon back to the atmosphere when fossil fuels are burned or weathering eventually brings them to the surface. The oceans also play a crucial role, exchanging CO2 with the atmosphere and hosting carbon in dissolved form and within marine organisms. This interconnected web explains carbon’s continual cycling on short and long timescales.

Other descriptions are narrower: focusing only on rock movement ignores the biological and atmospheric exchanges; plant–soil exchange excludes other reservoirs; and aquatic-only cycling misses land, atmosphere, and geological aspects integral to the cycle.