01The overall picture
6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂. Two stages: the light-dependent reactions (thylakoid membranes) capture light energy as ATP and NADPH; the Calvin cycle (stroma) uses them to build sugar from CO₂.
The O₂ released comes from splitting water, not from CO₂ — a classic exam question.
02Light reactions
Photosystem II absorbs light, exciting electrons in chlorophyll. Water is split to replace them, releasing O₂ and H⁺.
Electrons pass down an electron transport chain, which pumps H⁺ into the thylakoid lumen, building a gradient. H⁺ flows back out through ATP synthase, making ATP (chemiosmosis).
Photosystem I re-energizes the electrons, which reduce NADP⁺ to NADPH.
03Calvin cycle
Rubisco fixes CO₂ onto RuBP (carbon fixation). ATP and NADPH reduce the products into G3P, a 3-carbon sugar. Most G3P regenerates RuBP; some leaves to make glucose and other molecules.
Takes 3 CO₂ (3 turns) to export 1 G3P, using 9 ATP and 6 NADPH.
04Notebook box
Light reactions: IN water, light, NADP⁺, ADP → OUT O₂, ATP, NADPH. Calvin: IN CO₂, ATP, NADPH → OUT G3P, ADP, NADP⁺.
Worked example: A plant is given water with radioactive ¹⁸O. Where does it appear? → In the O₂ gas released.