AP® Biology · Unit 8 · Topic 8.2
The Carbon Cycle
The carbon cycle is the movement of carbon atoms between the atmosphere, organisms, and the ground, driven mainly by photosynthesis pulling carbon in and respiration pushing it back out.
Carbon never disappears. It just changes which molecule and which organism it's part of. A carbon atom can sit in atmospheric CO2, get pulled into a leaf, become part of a sugar, get eaten and built into your muscle, then get released again as you exhale. Matter cycles through ecosystems instead of flowing one way out, which is exactly why energy and matter behave so differently: energy enters as sunlight and leaves as heat, but the same carbon atoms get reused over and over.
Photosynthesis and cellular respiration are the two arrows that keep this cycle moving in opposite directions. Photosynthesis takes CO2 out of the air and fixes its carbon into glucose and other organic molecules inside producers. Respiration does the reverse: organisms break down those organic molecules for energy and release the carbon back out as CO2. Death and decomposition move carbon into the soil, where decomposers either respire it back into the air or, under rare conditions, let it settle into long-term storage as fossil fuel over millions of years.
That long-term storage is the catch. Coal, oil, and natural gas are carbon that fell out of the active cycle ages ago. Burning them pulls that carbon back into circulation as CO2, but far faster than photosynthesis and ocean absorption can pull it back out, so atmospheric carbon builds up. This links straight back to Unit 3: the same photosynthesis and respiration reactions you studied inside a single cell are the mechanisms running an entire ecosystem's carbon budget, scaled up from chloroplast to biosphere.
Key terms
- Carbon fixation
- The process of converting atmospheric CO2 into organic molecules, done by photosynthesis in producers.
- Cellular respiration
- The breakdown of organic molecules for energy that releases CO2 back into the atmosphere.
- Fossil carbon reservoir
- Carbon stored out of active circulation for millions of years in coal, oil, or natural gas.
- Decomposition
- The breakdown of dead organic matter by decomposers, which returns most of its carbon to the soil or atmosphere.
Watch it explained
- How it worksWalk me through carbon moving between the atmosphere, plants, animals, and soil.
- How it worksHow do photosynthesis and respiration act as opposite arrows in the same cycle?
- Going furtherWhat happens to atmospheric carbon dioxide when a fossil reservoir gets burned?
- Ties togetherHow does the carbon cycle connect the photosynthesis of Unit 3 to a whole ecosystem?
Check yourself
End of topic.