ROCKS — limestone (CaCO₃) formed from shells of ancient marine organisms.
Processes in the Carbon Cycle
REMOVING CO₂ FROM THE ATMOSPHERE:
PHOTOSYNTHESIS — plants and algae absorb CO₂ and convert it to glucose. Carbon is incorporated into the plant's biomass.
DISSOLUTION IN OCEANS — CO₂ dissolves in seawater to form carbonic acid. Marine organisms use dissolved carbon to make calcium carbonate (CaCO₃) shells.
RETURNING CO₂ TO THE ATMOSPHERE:
RESPIRATION — ALL living organisms (plants, animals, fungi, bacteria) respire, releasing CO₂ as glucose is broken down.
DECOMPOSITION — microorganisms (bacteria and fungi) break down dead organic matter, releasing CO₂ and returning carbon to the atmosphere and soil.
COMBUSTION — burning of organic material (wood, coal, oil, gas) releases CO₂ rapidly into the atmosphere.
FOSSILISATION — over millions of years, the remains of organisms become compressed into fossil fuels (coal from ancient forests, oil and gas from marine organisms).
Burning fossil fuels releases this long-term stored carbon rapidly — adding CO₂ to the atmosphere FASTER than natural processes can remove it → CLIMATE CHANGE.
Human Impact on the Carbon Cycle
Human activities are disrupting the carbon cycle by adding CO₂ to the atmosphere faster than it can be removed.
BURNING FOSSIL FUELS — coal, oil and natural gas burn to release CO₂ that has been stored underground for millions of years.
DEFORESTATION:
Felling trees removes photosynthesising plants → less CO₂ absorbed.
Burning or rotting felled trees releases CO₂.
Double negative impact on atmospheric CO₂.
CEMENT PRODUCTION — manufacturing cement releases CO₂ from limestone (CaCO₃ → CaO + CO₂).
RICE PADDY AGRICULTURE and LIVESTOCK — produce methane (CH₄), another greenhouse gas.
CONSEQUENCES OF RISING CO₂:
Enhanced greenhouse effect → global warming → climate change.
Ocean acidification — more CO₂ dissolves → more carbonic acid → threatens marine ecosystems (especially coral reefs and shell-forming organisms).
⚠️ Common Mistake
PLANTS also respire — they release CO₂ just like animals. The difference is that during the DAY, photosynthesis removes MORE CO₂ than respiration releases — so there is net CO₂ uptake. At night, only respiration occurs. Students often say 'plants absorb CO₂, they don't release it' — this is wrong.
📌 Key Note
CO₂ removed by: photosynthesis, dissolution in oceans. CO₂ returned by: respiration (all organisms), decomposition, combustion, volcanic activity. Fossil fuels = long-term carbon store. Burning them releases ancient carbon rapidly → climate change.