TL;DR:
Carbon moves between stores (air, living things, soil, oceans, rocks and fossil fuels) through flows such as photosynthesis, respiration, decay and burning.
Photosynthesis moves carbon from air into living matter. Respiration, decomposition and combustion move it back.
Fossil fuels are a very slow store. Burning them releases in a short time carbon that took a very long time to lock away.
Every protein, sugar and fat in your body has carbon in it. None of those atoms were made in your cells. They came from food, which came from plants, which took them from the air. Where does the carbon go next? The carbon cycle is the answer, and the easiest way to learn it is to follow a single atom.
The idea in plain words
Carbon is not used up. It moves between places called stores (or reservoirs) along paths called flows. The main stores are the atmosphere (as carbon dioxide), living things, soil and dead organic matter, the oceans, and rocks including fossil fuels.
A flow changes how much carbon is in each store. If more carbon enters a store than leaves, that store grows. The cycle is balanced when inflows and outflows match, and out of balance when they do not.
The main flows
Photosynthesis moves carbon from the atmosphere (as CO₂) into sugars in plants and algae.
Feeding passes that carbon from plants to animals and on to the next animal.
Respiration by plants, animals and microbes breaks down sugar and releases CO₂ back to the air.
Decomposition by fungi and bacteria turns dead matter into CO₂ or into soil carbon.
Dissolving moves CO₂ between the air and the sea surface, in both directions.
Burial can store carbon in sediments and, over very long periods, form coal, oil and gas.
Combustion burns wood and fossil fuels, releasing CO₂ quickly.
Worked example: following one carbon atom
Start with a carbon atom in a CO₂ molecule in the air above a field. Here is one possible path.
Air to leaf. A grass leaf takes in the CO₂ through its pores. In photosynthesis the carbon becomes part of a glucose molecule.
Leaf to rabbit. A rabbit eats the grass. The carbon is now in the rabbit's gut and then in its tissues.
Rabbit to air. The rabbit's cells use glucose in respiration. The atom leaves as CO₂ when the rabbit breathes out. The atom is back in the air.
Air to ocean. Carried by wind, the CO₂ dissolves in the sea surface. A shell-forming organism uses it to build a shell of calcium carbonate.
Ocean to rock. When the organism dies, its shell sinks and gets buried in sediment. Over a very long time the sediment becomes limestone. The atom is now in a slow store.
Rock to air. Eventually, through weathering or volcanic activity, the carbon returns to the air as CO₂, and the cycle continues.
This is only one of many possible paths. In another, the rabbit dies, a fungus decomposes it, and the carbon stays in soil for years before being respired. In a third, the grass is buried and slowly becomes coal. Each atom has its own route, but all the routes use the same flows.
Why fossil fuels matter in the cycle
Coal, oil and gas are the remains of ancient living things that were buried before they fully decomposed. The carbon in them was locked away for a very long time. When people burn fossil fuels, that carbon goes back into the air as CO₂ in a short time. The inflow to the atmosphere from combustion adds to the natural flows, so the atmosphere store grows.
This is why the carbon cycle sits at the centre of climate science: CO₂ in the atmosphere traps heat, so changing how much of it there is changes the climate. The cycle is the accounting system for how much is where.
Stores and flows with numbers
Treat each store like a bank balance. Say a forest holds 500 units of carbon (a made-up scale). Each year photosynthesis adds 40 units and respiration plus decay remove 38. The net flow is 40 − 38 = +2 units per year, so after five years the forest holds 500 + 5 × 2 = 510 units.
If the two flows were equal the store would stay level. If the forest burns, a large outflow happens in days, not years, and the store drops sharply. The same bookkeeping works for the atmosphere, the ocean or any other store.
How to check your answer
Name each store the atom is in, and the process that moved it between them.
A flow is a process (photosynthesis, respiration), and a store is a place (atmosphere, soil). Do not mix them.
Check that the carbon form is right: CO₂ in air, glucose in cells, calcium carbonate in shells, hydrocarbons in oil.
A complete loop should return to its starting store.
Common mistakes
Saying plants only photosynthesise and animals only respire. Plants respire too, day and night.
Saying carbon is used up when it is burned. It changes form, and the atoms remain.
Saying the ocean only takes carbon in. Exchange goes both ways.
Mixing up the carbon cycle with global warming. The cycle is a natural system. The warming comes from changes in its balance.
Where you will use it next
The same store and flow thinking is used for nitrogen, water and phosphorus, and for any model in biology that tracks quantities. To see where energy goes in the same living things, read food webs and energy in ecosystems. Energy flows through ecosystems and leaves as heat, while carbon cycles around and around.
Frequently asked questions
Is the carbon in my body really from the air?
Yes. Plants made it from CO₂ in the air, and you got it by eating plants or animals that ate them.
How long does carbon stay in each store?
It varies hugely. Carbon in a leaf might last months. Carbon in deep rock can stay for a very long time.
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