the Calvin cycle
/ KAL-vin SY-kuhl /
Once a solar charger has filled the batteries, you still need a workshop to spend that stored power on building something useful. The Calvin cycle is that workshop in photosynthesis. It takes the ATP and NADPH charged up by the light reactions and uses them to assemble sugar out of plain carbon dioxide from the air.
The Calvin cycle runs in the fluid interior of the chloroplast, the stroma. Its first key move is carbon fixation: an enzyme grabs carbon dioxide from the air and attaches it to a waiting molecule, turning inorganic carbon into part of an organic chain. Then, powered by ATP and NADPH from the light reactions, a series of steps reshapes and reduces these molecules into a small sugar. Crucially, the cycle also regenerates its own starting molecule so it can grab the next carbon dioxide, going round and round. It takes several turns and many carbon dioxide molecules to build one molecule of glucose-sized sugar.
This is the step where air literally becomes food and body: the carbon in a plant, and ultimately in you, was pulled from the atmosphere here. The Calvin cycle is often called the "dark reactions," but that name misleads — it does not require darkness and usually runs in daylight. It is light-independent only in the narrow sense that its own steps use no light directly; it still depends utterly on the ATP and NADPH that the light reactions supply, so it stalls without light.
The carbon in a wooden table started as carbon dioxide gas in the air; the Calvin cycle in a tree's leaves fixed that carbon into sugar, which the tree then built into wood.
The Calvin cycle fixes airborne carbon dioxide into sugar, spending the ATP and NADPH from the light reactions.
"Dark reactions" is a misleading name: the Calvin cycle does not need darkness and normally runs by day. It uses no light directly but stalls without it, since it depends on the light reactions' ATP and NADPH.