The carbon cycle (spec 4.12)
Photosynthesis takes COβ out; respiration, decomposition, combustion put it back.
The carbon cycle moves carbon atoms between the atmosphere, oceans, living organisms and fossil fuels. The atmosphere contains ~0.04 % COβ β small but vital.
Processes that REMOVE COβ from the atmosphere:
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Photosynthesis (the ONLY major sink). Plants, algae and some bacteria absorb COβ β fix it into glucose using light energy. The carbon now enters the food chain.
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Ocean dissolution. COβ dissolves in seawater forming carbonic acid (HβCOβ) and bicarbonate ions (HCOββ»). Marine producers (phytoplankton) use this dissolved COβ. Some carbon is locked into carbonate shells (forams, corals) β eventually limestone.
Processes that RETURN COβ to the atmosphere:
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Respiration. All living organisms (plants, animals, decomposers) respire continuously: glucose + Oβ β COβ + HβO + energy. This is the main natural source of atmospheric COβ.
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Decomposition. When organisms die, saprobiotic bacteria and fungi break down the dead matter. As they respire the absorbed nutrients, they release COβ.
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Combustion (burning). Burning wood OR fossil fuels (coal, oil, natural gas) releases COβ. Fossil fuels are the compressed remains of plants and animals that died millions of years ago in anaerobic conditions β burning them releases carbon that has been locked away geologically.
Carbon flow up the food chain (feeding):
- Plants fix COβ β glucose, then convert glucose to starch, cellulose, proteins, lipids, DNA β all organic carbon compounds.
- Herbivores eat plants β digest carbon compounds β use for their own tissues + respiration.
- Carnivores eat herbivores β same process.
- All organisms eventually die β decomposers break them down β COβ returned to atmosphere.
Long-term lock-up:
- Fossil-fuel formation: dead organisms buried in anaerobic conditions (peat bogs, ocean sediments) over millions of years β coal, oil, natural gas. Carbon locked out of the active cycle.
- Limestone: carbonate shells of marine organisms accumulate on the seabed β become sedimentary rock. Released only by weathering or volcanism over geological timescales.
Human disruption. In the natural cycle, photosynthesis and respiration roughly balance. Burning fossil fuels and deforestation add COβ faster than photosynthesis can remove it β atmospheric COβ has risen from 280 ppm (pre-industrial) to >420 ppm today β ENHANCED greenhouse effect β global warming.
- Photosynthesis = ONLY major process removing COβ.
- Respiration + decomposition + combustion return COβ.
- Fossil fuels = ancient carbon locked away; burning returns it.
- Carbon flows up food chain via feeding; returns via respiration / decomposition.
- Human disruption: combustion + deforestation outpace photosynthesis β COβ rise.
See the full worked example for cycles within ecosystems β