How does carbon move between living organisms, the atmosphere, oceans and rocks?
Topic 1.4 The Carbon Cycle: describe the major reservoirs and fluxes of the carbon cycle and explain how natural processes and human activities move carbon between them.
A focused answer to APES Topic 1.4, covering carbon reservoirs and fluxes, photosynthesis and respiration, decomposition, combustion, the ocean as a carbon sink, and how fossil fuel burning alters the cycle, with a worked carbon-flux calculation.
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What this topic is asking
The College Board (Topic 1.4) wants you to describe the carbon cycle: where carbon is stored (its reservoirs), how it moves between them (its fluxes), and how human activity has changed those flows. The biological core is the balance between photosynthesis (which removes carbon dioxide) and cellular respiration (which releases it), set against the geological store of carbon in rocks and fossil fuels.
Reservoirs and fluxes
A reservoir that mainly stores carbon for long periods is called a sink; one that mainly releases it is a source. The same reservoir can act as either depending on conditions.
The biological fluxes
When these are in balance, atmospheric carbon dioxide stays roughly steady. Photosynthesis slightly exceeding respiration over time is what built up Earth's stores of organic carbon.
Long-term storage and combustion
Carbon is locked away for long periods in two main ways. Fossil fuels (coal, oil and natural gas) form when dead organisms are buried under anaerobic conditions and compressed over millions of years, storing their carbon underground. Carbonate rocks such as limestone store carbon from the shells of marine organisms. Combustion, the burning of fossil fuels or biomass, rapidly releases this stored carbon as carbon dioxide.
The ocean and human impact
The ocean is a major carbon sink: carbon dioxide dissolves into surface water, where it is used by marine producers and incorporated into shells and sediments. This uptake slows the rise of atmospheric carbon dioxide but causes ocean acidification as dissolved carbon dioxide forms carbonic acid.
Human activity has disturbed the balance of the cycle. Burning fossil fuels transfers carbon from a long-term geological store into the atmosphere in decades rather than over the millions of years it took to form. Deforestation removes the photosynthesising trees that would otherwise absorb carbon dioxide, and burning or decomposing the cleared vegetation releases yet more. Because these fluxes now add carbon dioxide to the atmosphere faster than photosynthesis and ocean uptake can remove it, atmospheric carbon dioxide has risen sharply, enhancing the greenhouse effect. The carbon cycle is thus the central link between Unit 1's ecosystem science and the later units on pollution and climate change: every tonne of fossil carbon burned is a flux out of a sink that took geological time to fill.
Try this
Q1. Identify the process that returns carbon from dead organisms to the atmosphere. [1 point]
- Cue. Decomposition (carried out by decomposers; releases carbon dioxide).
Q2. Explain why burning fossil fuels raises atmospheric carbon dioxide. [2 points]
- Cue. Combustion releases carbon that was stored underground for millions of years, adding carbon dioxide to the atmosphere faster than photosynthesis and the ocean can remove it.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2021 (style)4 marksSection II (FRQ). (a) Identify the two biological processes that move carbon between the atmosphere and living organisms, and state the direction of carbon movement in each. (b) Describe how carbon is stored long-term in fossil fuels. (c) Explain how human activity has increased atmospheric carbon dioxide. (d) Describe one role the ocean plays in the carbon cycle.Show worked answer →
A 4-point FRQ on carbon fluxes and human impact.
(a) Identify (1 point): photosynthesis removes carbon dioxide from the atmosphere and fixes it into organic compounds (atmosphere to organisms); cellular respiration releases carbon dioxide back to the atmosphere (organisms to atmosphere).
(b) Describe (1 point): when organisms die and are buried under anaerobic conditions over millions of years, their carbon-rich remains are compressed and transformed into fossil fuels (coal, oil, natural gas), a long-term carbon store.
(c) Explain (1 point): burning fossil fuels (combustion) and deforestation release stored carbon as carbon dioxide faster than natural sinks remove it, raising atmospheric carbon dioxide.
(d) Describe (1 point): the ocean acts as a carbon sink, dissolving atmospheric carbon dioxide; marine organisms and chemistry store carbon, though this also causes ocean acidification.
Markers reward naming both processes with correct directions, linking fossil fuels to buried organic matter, attributing rising carbon dioxide to combustion and deforestation, and identifying the ocean as a sink.
AP 2019 (style)1 marksSection I (multiple choice). Which process transfers carbon from the atmosphere into producers? (A) Cellular respiration (B) Combustion (C) Photosynthesis (D) Decomposition. Justify your choice.Show worked answer →
A 1-point MCQ on carbon fluxes. The answer is (C).
Photosynthesis takes carbon dioxide from the atmosphere and converts it into glucose in producers, moving carbon from the atmosphere into living matter. Respiration (A), combustion (B) and decomposition (D) all release carbon dioxide back to the atmosphere. The trap is reversing the direction of photosynthesis and respiration.
Related dot points
- Topic 1.5 The Nitrogen Cycle: describe the steps of the nitrogen cycle and explain how nitrogen fixation, the role of bacteria and human activities move nitrogen between reservoirs.
A focused answer to APES Topic 1.5, covering nitrogen fixation, nitrification, assimilation, ammonification and denitrification, the central role of bacteria, and how synthetic fertilizer alters the cycle, with a worked nitrogen-input question.
- Topic 1.6 The Phosphorus Cycle: describe the phosphorus cycle, explain why it has no significant atmospheric component, and explain how phosphorus acts as a limiting nutrient and a pollutant.
A focused answer to APES Topic 1.6, covering the slow sedimentary phosphorus cycle, weathering and uptake, why there is no gas phase, phosphorus as a limiting nutrient, and how mining and detergents cause eutrophication, with a worked limiting-nutrient question.
- Topic 1.7 The Hydrologic (Water) Cycle: describe the processes of the water cycle and explain how human activities alter the storage and movement of water.
A focused answer to APES Topic 1.7, covering evaporation, transpiration, condensation, precipitation, runoff, infiltration and groundwater, and how deforestation, paving and irrigation alter the cycle, with a worked water-budget calculation.
- Topic 1.8 Primary Productivity: define gross and net primary productivity, explain the factors that control them, and calculate net primary productivity from data.
A focused answer to APES Topic 1.8, covering gross and net primary productivity, respiration, the GPP-NPP relationship, limiting factors, productivity across biomes, and ecological efficiency, with a full worked NPP calculation.
- Topic 1.9 Trophic Levels: describe the trophic levels of an ecosystem and explain the roles of producers, consumers and decomposers in transferring energy and matter.
A focused answer to APES Topic 1.9, covering producers, primary, secondary and tertiary consumers, decomposers and detritivores, autotrophs and heterotrophs, and how energy and matter move through trophic levels, with a worked classification question.
Sources & how we know this
- AP Environmental Science Course and Exam Description — College Board (2020)