Describe the carbon cycle
WHAT is the carbon cycle?
The carbon is held in stores (reservoirs) and moved by fluxes (processes):
| Store | Form of carbon |
|---|---|
| Atmosphere | carbon dioxide gas (CO₂), plus some methane (CH₄) |
| Living organisms | carbohydrates, fats, proteins |
| Oceans | dissolved CO₂, hydrogencarbonate (HCO₃⁻), carbonate (CO₃²⁻) ions, and particulate organic carbon (plankton, detritus) |
| Fossil fuels | coal, oil, natural gas |
| Rocks | carbonate rock (e.g. limestone, chalk = CaCO₃) |
WHY does carbon cycle at all?
HOW carbon moves — the biological fluxes
Think of it as IN to living things and OUT of living things.
Feeding transfers carbon along food chains (plant → herbivore → carnivore) as organic molecules.
Decomposition (aerobic): when organisms die with oxygen present, decomposers (bacteria & fungi) feed on the dead matter and respire, returning carbon to the air as CO₂. Without them, carbon would be trapped in corpses.
Combustion (burning): burning wood or fossil fuels releases stored carbon as CO₂:
Fossil fuel formation: if dead matter is buried before decomposers reach it (anoxic), over millions of years pressure and heat turn it into coal, oil and gas.
HOW carbon moves — the ocean and rock (geological) fluxes
Dual-coding: reading the diagram
- Arrows UP into the air = release CO₂/CH₄ → respiration, combustion, decomposition, volcanoes, ocean outgassing.
- Arrows DOWN out of air = removal → photosynthesis, ocean dissolution, rock weathering.
- Sideways arrows = feeding/death (carbon stays organic, just changes organism).
- Long slow loop = shells → sediment → rock → subduction → volcano.
Worked reasoning examples
Common mistakes (steel-manned)
Forecast-then-verify
Recall Predict before revealing
Q: If atmospheric CO₂ suddenly doubled, name TWO natural processes that would start removing the excess — and say which is faster. Predict... Answer: (1) Ocean dissolution — more CO₂ dissolves into seawater (fast, years–decades). (2) Rock weathering — carbonic acid weathers silicate rock (very slow, thousands+ years). Dissolution acts sooner but has limited capacity; weathering is the ultimate long-term sink.
Flashcards
What process removes CO₂ from the atmosphere biologically?
Word equation for photosynthesis?
Which processes return CO₂ to the atmosphere?
What gas is produced by decomposition in oxygen-free conditions?
What organisms carry out aerobic decomposition?
In which two main dissolved forms is carbon stored in the ocean?
How does rock weathering affect atmospheric CO₂?
How is carbonate rock (limestone) formed?
How is buried carbonate carbon returned to the air?
How are fossil fuels formed?
Which two reservoirs hold the most carbon?
Which human activity adds extra CO₂ and unbalances the cycle?
Recall Feynman: explain to a 12-year-old
Imagine carbon atoms are little Lego bricks. There's only a certain number on Earth. Plants pull bricks out of the air (photosynthesis) and build them into food. Animals eat the food and take the bricks. When anything breathes out, or a bug rots a dead leaf, the bricks float back into the air — and in a swamp with no air, the bricks come out as a different gas called methane. Some bricks dissolve into the sea; sea creatures use them to build shells, and those shells turn into rock (like chalk) that hangs onto the bricks for millions of years — until a volcano finally burps them back out. So the same bricks keep getting reused, some fast, some super slow. The problem: when we dig up coal and oil and burn it, we dump loads of extra old bricks into the air all at once, and that warms the planet.
Connections
- Photosynthesis — the main biological CO₂ removal step
- Respiration — the reverse process, main CO₂ release
- Decomposers and decay — recycling carbon; aerobic (CO₂) vs anaerobic (CH₄)
- Food chains and energy flow — how carbon moves between organisms
- Ocean acidification and carbonate chemistry — the seawater CO₂/HCO₃⁻/CO₃²⁻ equilibrium
- Rock cycle and weathering — geological carbon drawdown and return
- The greenhouse effect and climate change — what happens when the cycle is unbalanced
- Fossil fuels — long-term carbon store
- Nitrogen cycle — a parallel nutrient cycle to compare
Concept Map
Hinglish (regional understanding)
Intuition Hinglish mein samjho
Dekho, carbon cycle ka matlab hai ki carbon atoms Earth pe ghoomte rehte hain — kabhi hawa mein CO₂ ki tarah, kabhi plant ke andar glucose ki tarah, kabhi animal ke body mein, kabhi samundar mein ghula hua, aur kabhi coal-oil ya chattan (limestone) ki tarah zameen ke neeche. Carbon kabhi khatam nahi hota, bas form aur jagah badalta rehta hai. Do speed ke cycle hote hain: ek fast biological (photosynthesis ↔ respiration, din-saal) aur ek slow geological (ocean chemistry, rock weathering, sedimentation, volcano — millions of years).
Biological side pe: photosynthesis carbon ko hawa se andar laata hai, aur respiration, decomposition, combustion wapas hawa mein bahar bhejte hain. Ek zaroori point — decomposition hamesha CO₂ nahi deta. Jahan oxygen nahi hoti (bog,