The carbon cycle
Chemistry · Key Stage 3 · The School
Carbon moves, it does not vanish
The same carbon atoms move endlessly between four places: the air, as carbon dioxide gas; living things, as part of sugars and proteins; the ground, locked in rock, soil or fossil fuels; and back to the air again. Nothing about carbon disappears at any stage. It only changes which form it is currently in.
Photosynthesis and respiration
PHOTOSYNTHESIS takes carbon dioxide out of the air and locks the carbon into a plant's sugars, using energy from sunlight. RESPIRATION does the reverse: living things break down those same sugars for energy, releasing carbon dioxide back into the air. These two processes constantly move carbon in opposite directions, keeping the cycle turning.
Decomposition and fossil fuels
When a plant or animal dies, decomposers break it down, and most of its carbon returns to the air as carbon dioxide fairly quickly. But sometimes dead material gets buried before it can fully decompose, and over millions of years, immense heat and pressure turn it into coal, oil or natural gas — FOSSIL FUELS, carbon locked away from the fast cycle for an extremely long time.
Suppose a log has 10 kg of wood in it, with the water dried out. Wood is about half carbon, so the log holds about 5 kg of carbon. Burn it completely and that carbon joins oxygen from the air to make carbon dioxide. Each carbon dioxide molecule has one carbon atom and two oxygen atoms. Scientists give each atom a number that compares masses. Carbon is 12 and oxygen is 16. One carbon dioxide molecule adds up to 12 + 16 + 16 = 44. So carbon and carbon dioxide are always in the ratio 12 to 44, in any unit. 12 g of carbon makes 44 g of carbon dioxide. That means 5 kg of carbon makes 5 × 44 ÷ 12 = about 18 kg of carbon dioxide. The tree took in that same 18 kg from the air as it grew. That took decades, and the log gives it back in an hour. Over its whole life and its burning, the air ends up where it began. Now suppose some coal holds the same 5 kg of carbon. Burning it also makes about 18 kg of carbon dioxide, very quickly. This time nothing recent was taken from the air to match it. The plants that took in this carbon did so millions of years ago, and it stayed locked away until it was burned. The fast cycle gains 18 kg of carbon dioxide, all at once.