Photosynthesis
Light reactions occur in thylakoid membranes. Photosystem II supplies electrons by oxidizing water, releasing O₂. Electron transport builds a high H⁺ concentration in the lumen; protons return to the stroma through ATP synthase. Photosystem I helps reduce NADP⁺. The Calvin cycle occurs in the stroma, fixes CO₂ using rubisco, reduces carbon using ATP and NADPH, and regenerates RuBP. Its net exported carbohydrate is G3P. Cyclic electron flow around photosystem I can produce ATP without NADPH or O₂.
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A labeled oxygen atom in water can appear in released O₂; labeled carbon from CO₂ can appear in carbohydrate.
DON’T FALL FOR THIS
The Calvin cycle does not run indefinitely in darkness: it needs light-reaction products.