Light-dependent reactions
Chloroplasts have thylakoids (stacked into grana) where the light-dependent reactions occur, surrounded by stroma (light-independent). Photosystems contain chlorophyll a and b and accessory pigments (carotenoids) that absorb different wavelengths; the absorption spectrum matches the action spectrum. Non-cyclic photophosphorylation: light excites electrons in chlorophyll (photoionisation) of PSII; they pass along an electron transport chain to PSI, then to NADP, forming reduced NADP; energy pumps H⁺ into the thylakoid space and ATP synthase makes ATP by chemiosmosis. Photolysis of water replaces PSII's electrons: . Cyclic photophosphorylation (PSI only) makes ATP but no reduced NADP or O₂. Pigments are separated by chromatography ().
≈ 50 minCalvin cycle and limiting factors
The Calvin cycle (stroma) uses ATP and reduced NADP from the light-dependent stage. Fixation: RuBisCO combines CO₂ with ribulose bisphosphate (RuBP, 5C) to form two glycerate 3-phosphate (GP, 3C). Reduction: GP → triose phosphate (TP) using ATP and reduced NADP. Regeneration: most TP (5 of every 6) is used to remake RuBP using ATP. TP is used to make glucose, sucrose, starch, cellulose, lipids and amino acids. Six turns fix six CO₂ to make one hexose. Limiting factors: light intensity, CO₂ concentration and temperature; the factor in shortest supply limits the rate. In the dark, GP accumulates and RuBP falls; with low CO₂, RuBP accumulates and GP falls. C4 plants (maize) fix CO₂ in mesophyll cells and release it to bundle-sheath cells, keeping RuBisCO away from O₂.
≈ 50 min