The carbon cycle: matter moving through life and Earth — Biology, 14–17 years
Carbon atoms move between the air, living things, soils, oceans, and rocks. Photosynthesis, respiration, decomposition, and combustion change where carbon is stored.
Idea
The carbon cycle follows carbon atoms as they move through air, water, rocks, soil, and living things. Plants take carbon dioxide from the air and build it into organic molecules. Animals receive some carbon by feeding, while respiration, decomposition, and combustion return carbon dioxide or other carbon compounds to the surroundings.
Why it matters
Life needs carbon to build molecules such as sugars, fats, proteins, and DNA, but the amount of carbon dioxide in the atmosphere also affects Earth’s temperature. The cycle shows that carbon is not destroyed when wood or fuel burns; it changes form and moves location. Human activities can transfer stored carbon to the air faster than natural processes remove it.
Worked example
A tree takes in one carbon dioxide molecule during photosynthesis. Its carbon becomes part of a leaf, then a caterpillar eats the leaf and uses that carbon in its body. When the caterpillar respires, some carbon leaves as carbon dioxide; when it dies, decomposers return more carbon compounds to the soil and air.
Common trap
A reasonable mistake is to think plants remove carbon dioxide permanently from the atmosphere. Plants do remove it during photosynthesis, but they also respire, and their stored carbon returns through feeding, decay, or burning. A forest can hold a lot of carbon without locking it away forever.
Outside school
Climate scientists measure carbon moving between forests, oceans, soils, and the atmosphere to understand climate change. Farmers use the cycle when managing soil carbon, crop remains, and fertilisers. Governments also use carbon budgets to compare emissions with the amount that forests, soils, or technology can remove from the air.
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