27.8
The Carbon Cycle
Carbon is the basis of all organic matter on Earth, and is recycled through the ecosystem in two primary processes: one in which carbon is exchanged a…
In the carbon cycle, carbon atoms travel throughout the ecosystem.
Starting with photosynthesis, autotrophs, such as plants, transform carbon from the atmosphere in the form of carbon dioxide into consumable energy that is ingested by heterotrophs. These organisms can then return the carbon into the atmosphere through respiration or into the soil during decomposition.
Carbon dioxide in the atmosphere is also highly soluble in water so the oceans constitute a major reservoir of carbon.
Carbon dioxide reacts with water to form carbonic acid and then carbonate ions. Calcium carbonate in ocean water is used to build coral skeletons and mollusc shells. Carbon can also be stored for short terms in the bodies of long-lived trees and in the soil or, for a much longer time in the case of fossil fuels, the organic remains of once living organisms.
Finally, carbon stored in sediments and soils of the earth's crust can be melted into magma and vented into the atmosphere during volcanic eruptions, a process called degassing.
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Q1: How do plants and animals exchange carbon in the carbon cycle?
Plants absorb carbon dioxide from the atmosphere during photosynthesis and convert it into organic compounds like glucose. Heterotrophs consume these plants to obtain carbon for growth and energy. Both organisms return carbon to the atmosphere through cellular respiration, which releases carbon dioxide as a byproduct, completing the biological exchange.
Q2: What role do decomposers play in returning carbon to the environment?
Decomposers such as bacteria and fungi break down dead organisms and animal waste into carbon dioxide and organic compounds. This decomposition process releases carbon back into the atmosphere and soil, allowing it to re-enter the ecosystem and be used by other organisms for growth and energy.
Q3: How does carbon dioxide interact with ocean water?
Carbon dioxide is highly soluble in water and reacts with water molecules to form carbonic acid and carbonate ions. These carbonates combine with calcium ions to create calcium carbonate, which marine organisms like coral and oysters use to build their shells and skeletons, making oceans a major carbon reservoir.
Q4: Where is carbon stored for long periods in the Earth?
Carbon is stored long-term in fossil fuels like coal, petroleum, and natural gas deep within the Earth, formed from organic remains over millions of years. Additionally, limestone in ocean sediments and large, long-lived trees store carbon for extended periods. These geological reservoirs represent the largest carbon sinks on Earth.
Q5: How do volcanic eruptions release carbon back into the atmosphere?
Carbon stored deep under the Earth's surface in sediments and soils can be melted into magma. During volcanic eruptions, this process called degassing vents carbon dioxide directly into the atmosphere, returning carbon that has been stored in the Earth's crust for extended periods.
Q6: What human activities are disrupting the natural carbon cycle?
Burning fossil fuels and wood releases substantial amounts of carbon dioxide into the atmosphere. Deforestation and increased agricultural practices reduce organic matter in soil, decreasing carbon storage and increasing greenhouse gases like methane, fundamentally altering the balance of what are biogeochemical cycles.
Q7: Why are oceans considered a major carbon reservoir?
Oceans absorb carbon dioxide from the atmosphere, which dissolves and reacts with water to form carbonates. When marine organisms die, their calcium carbonate shells and skeletons sink to the ocean floor and gradually accumulate as sediment, eventually forming limestone, the largest carbon reservoir on Earth.