Biological importance: Carbon forms the backbone for all organic molecules.

Forms available to life: Photosynthetic organisms convert CO₂ to organic forms used by consumers.

Reservoirs: Soils, aquatic sediments, oceans, plant and animal biomass, and the atmosphere. The largest reservoir is sedimentary rocks such as limestone, though this pool turns over very slowly.

Key processes: Photosynthesis removes CO₂ from the atmosphere is balanced by CO₂ released through cellular respiration.

The burning of fossil fuels adds significant amounts of additional CO₂.

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