Energy flows through an ecosystem and is lost, but the atoms that build living things are used over and over. Carbon moves between the air, organisms and fossil fuels; nitrogen moves between the air, the soil and proteins, with microorganisms doing most of the work. The nitrogen cycle is Extended content.
In the carbon cycle, photosynthesis removes carbon dioxide from the air; feeding passes carbon compounds along food chains; respiration by plants, animals and decomposers and combustion of fossil fuels return carbon dioxide to the air; dead material that does not decompose can form fossil fuels over millions of years. Extended: in the nitrogen cycle microorganisms carry out decomposition, nitrification, nitrogen fixation and denitrification, while plants absorb nitrate ions to make amino acids and proteins.
Carbon atoms (the black dots) travel along every arrow. Click one process at a time to follow it on its own, then look at which processes take carbon dioxide out of the air and which put it back.
| Process | What happens to the carbon | Effect on CO2 in the air |
|---|---|---|
| Photosynthesis | plants take in carbon dioxide and make glucose and other carbon compounds | removes |
| Feeding | carbon compounds in plants pass to animals that eat them, and on along the food chain | no direct change |
| Respiration | plants, animals and decomposers break down glucose and release carbon dioxide | adds |
| Decomposition | decomposers (bacteria and fungi) feed on dead organisms and waste, then respire | adds |
| Formation of fossil fuels | dead material that does not fully decay (for example buried without oxygen) becomes coal, oil and gas over millions of years | locks carbon away |
| Combustion | burning fossil fuels (and wood) releases carbon dioxide | adds |
Plants respire all the time, day and night, as well as photosynthesising. Do not draw respiration arrows only from animals; plants and decomposers release carbon dioxide too.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the carbon cycle simulation, which process removes carbon dioxide from the air?
Which organisms release carbon dioxide by respiration in the carbon cycle sim?
Extended: in the nitrogen cycle sim, which process do nitrifying bacteria carry out?
Extended: where does the sim show denitrifying bacteria working best?
Extended. Most nitrogen is nitrogen gas in the air, which plants and animals cannot use. Click each process to see how nitrogen gets into proteins and back out again, and which steps depend on microorganisms.
| Microorganism role | Converts |
|---|---|
| Decomposition | protein and urea → ammonium ions |
| Nitrification | ammonium ions → nitrate ions |
| Nitrogen fixation | nitrogen gas → ammonium compounds |
| Denitrification | nitrate ions → nitrogen gas |
Nitrification and nitrogen fixation are different processes. Fixation starts from nitrogen gas; nitrification starts from ammonium ions already in the soil. Denitrification is the one that returns nitrogen gas to the air.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
How can carbon from dead plants become locked away for millions of years?
Which process adds carbon dioxide to the air but is not carried out by living organisms?
Extended: what is deamination?
Extended: a farmer grows beans and then ploughs them into the soil. Why does the next crop grow better?
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