Carbon on the Move: Photosynthesis, Respiration, and Ecosystems

Students construct and interpret a model showing how photosynthesis, cellular respiration, decomposition, and human activities move carbon through an ecosystem.

Carbon Reservoirs in Earth Systems

Carbon moves among four major Earth systems. The atmosphere stores carbon mainly as carbon dioxide and methane. The biosphere holds carbon in living organisms, dead matter, and soils. The hydrosphere contains dissolved carbon dioxide, bicarbonate, and carbon in aquatic organisms. The geosphere stores enormous amounts of carbon in rocks, sediments, and fossil fuels. These locations are called reservoirs because carbon can remain in them for different lengths of time. Carbon may stay in a leaf for months, in soil for centuries, or in limestone for millions of years. For example, atmospheric carbon dioxide can dissolve in ocean water, become part of a shell, and eventually enter seafloor sediment. Reservoir size and storage time help scientists explain how carbon moves through ecosystems and affects climate.

Photosynthesis and Carbon Storage

Photosynthesis moves carbon from the atmosphere or water into the biosphere. Plants, algae, and some bacteria use light energy to combine carbon dioxide and water, producing energy-rich sugars and oxygen. The carbon atoms from carbon dioxide become part of glucose and other organic molecules. Organisms use these molecules to build leaves, roots, wood, fats, and proteins. This process stores carbon in biomass until it is transferred, released, or buried. For example, a growing oak tree removes carbon dioxide from the air and stores much of that carbon in its trunk, branches, roots, and surrounding soil. Some stored carbon enters food webs when herbivores eat plant tissue. Photosynthesis does not destroy carbon; it changes carbon-containing molecules and transfers carbon from one reservoir to another.

Respiration and Decomposition

Cellular respiration returns carbon from organic molecules to the atmosphere or hydrosphere. Most organisms, including plants, animals, fungi, and many microorganisms, break down sugars using oxygen and release carbon dioxide, water, and usable energy. Respiration occurs continuously, not only when animals breathe. Decomposition is carried out mainly by bacteria, fungi, and detritivores that consume dead organisms and wastes. As decomposers respire, they release carbon dioxide; in low-oxygen environments, some microorganisms also produce methane. Some carbon remains in soil as organic matter or becomes buried in sediments. For example, when a fallen leaf is decomposed, part of its carbon enters fungal biomass, part is released as carbon dioxide, and part remains in the soil. Respiration and decomposition therefore connect living organisms, the atmosphere, water, and soil reservoirs.

Modeling Carbon Transfers

A carbon-cycle model should distinguish reservoirs, which store carbon, from processes, which transfer it. Boxes can represent the atmosphere, organisms, soil, oceans, rocks, and fossil fuels. Labeled arrows can represent photosynthesis, feeding, respiration, decomposition, ocean exchange, burial, combustion, and weathering. Arrow direction shows where carbon moves, while arrow width may represent the relative rate of transfer. Reservoir size can be shown by box size, but the model must include a key explaining this choice. For example, a wide arrow from the atmosphere to plants during the growing season can represent rapid photosynthetic uptake, while a thin arrow into sediment can represent slower burial. Scientists compare such models with satellite measurements, atmospheric records, field observations, and ocean data. A useful model simplifies reality while preserving the processes needed to answer a scientific question.

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Human Activities and Carbon Balance

Human activities alter carbon transfers and can change the balance among reservoirs. Burning coal, oil, and natural gas rapidly moves carbon from the geosphere to the atmosphere as carbon dioxide. Cement production also releases carbon dioxide, while deforestation reduces carbon storage in biomass and may release carbon through burning and decomposition. Political and economic decisions influence where and how strongly these changes occur. For example, a government may subsidize fossil-fuel extraction, increasing employment and energy production while also increasing emissions and environmental impacts. Alternatively, policies may support renewable energy, forest protection, public transportation, or wetland restoration. These choices can reshape local economies, land use, air quality, and ecosystem health. Oceans and plants absorb some human-produced carbon dioxide, but not all of it. The remaining increase in atmospheric carbon dioxide contributes to climate change and ocean acidification.

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Carbon-Cycle Model Check

A strong carbon-cycle model must conserve matter: carbon atoms move or change chemical form, but they are not created or destroyed. Check that every arrow begins in one reservoir and ends in another, and label each arrow with a process rather than a substance. Photosynthesis should move carbon into producers, while respiration should move carbon dioxide out of organisms. Decomposition should connect dead matter with decomposers, soil, and the atmosphere or water. Human combustion should transfer carbon from fossil fuels or biomass to the atmosphere. Then compare the model with evidence from different formats, such as a carbon dioxide graph, a satellite vegetation map, and an emissions table. For example, seasonal decreases in atmospheric carbon dioxide should agree with increased plant growth in the Northern Hemisphere. If evidence conflicts with the model, revise the arrows, rates, or reservoir sizes and explain why.

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