The Carbon Cycle

Carbon never sits still. A carbon atom in CO₂ gets pulled out of the air by a plant during photosynthesis, built into a sugar molecule, and becomes part of a leaf. An animal eats the leaf, and the carbon becomes part of the animal. Eventually respiration breaks that sugar back down for energy, releasing the carbon as CO₂ again. Photosynthesis and respiration are the same reaction run in opposite directions: one takes in CO₂ and sunlight to build carbon-based molecules and release O₂, the other burns those molecules with O₂ to release energy and CO₂. Together they form the fast loop that cycles most carbon on a yearly scale.

Not all carbon stays in that fast loop. Some gets dissolved into the ocean, some gets buried in sediment, shells, or dead organic matter that never fully decomposes, and over millions of years that buried carbon can turn into coal, oil, or natural gas. That slow storage is what makes fossil fuel carbon different: it dropped out of the active cycle long ago. Burning it now does not just recirculate carbon that was already moving between plants, animals, and air — it adds brand new carbon to that active system, on top of what was already flowing through it.

This cycle connects directly to the energy story of Unit 1. Energy flows one way through an ecosystem and mostly leaves as heat, but carbon loops endlessly between pools: atmosphere, biosphere, ocean, and crust. How long a carbon atom stays in a given pool before moving on is its residence time, and comparing residence times is how the fast biological cycle and the slow geological cycle get tied together.