The Calvin Cycle

The Calvin cycle runs in the stroma of the chloroplast and has three jobs: fix carbon, reduce it, and regenerate the molecule that catches the next CO2. Fixation happens when the enzyme rubisco attaches a CO2 to a five-carbon sugar called RuBP, making an unstable six-carbon compound that splits into two three-carbon molecules (3-PGA). Reduction comes next: ATP adds a phosphate to each 3-PGA, then NADPH donates electrons, turning them into G3P, a higher-energy three-carbon sugar. Most of that G3P gets recycled through a series of rearrangements that rebuild RuBP, spending more ATP, so the cycle can fix another CO2. Only a fraction of the G3P actually leaves the cycle to build glucose and other organic molecules.

The ATP and NADPH spent in reduction and regeneration are both made in the thylakoid during the light-dependent reactions, then diffuse into the stroma. That's the link between the two halves of photosynthesis: light reactions split water, pass electrons down a chain, and use the proton gradient to make ATP, while also loading NADP+ into NADPH. The Calvin cycle has no direct use for light itself, which is why it is called light-independent, but it cannot run for long without that steady delivery of ATP and NADPH, which is why it stalls in the dark.

Because rubisco fixes one CO2 per turn, building one glucose, a six-carbon sugar, takes six turns of the cycle, consuming 18 ATP and 12 NADPH along the way. On a hot day a plant may close its stomata to save water, which also blocks CO2 from entering. Rubisco then starts fixing O2 instead in a wasteful process called photorespiration, and sugar production drops even though light and the light reactions keep running fine.