Q.Write in detail the three main stages of photosynthesis.
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Start your 14-day free trial to unlock the full solution →Photosynthesis is not a single step but a sequence: light energy is first captured and converted into chemical energy carriers (ATP, NADPH) in the thylakoid membranes, and that chemical energy is then used, independent of light, to fix atmospheric CO₂ into sugar in the stroma.
Stage 1 — Light (photochemical) reactions: Occurring in the thylakoid membranes of the chloroplast, chlorophyll and accessory pigments absorb light energy, exciting electrons in Photosystem II. This drives the photolysis of water (splitting H₂O into O₂, protons, and electrons), releasing oxygen as a byproduct. The excited electrons pass onward, ultimately reducing NADP⁺ to NADPH.
Stage 2 — Electron transport chain and photophosphorylation: As excited electrons move through a chain of membrane-embedded carriers (between Photosystem II and Photosystem I, and onward), the energy released pumps protons across the thylakoid membrane, building a proton gradient. This gradient drives ATP synthase to produce ATP from ADP and inorganic phosphate — a process called photophosphorylation. Together, Stages 1 and 2 are often described jointly as "the light reaction," producing the ATP and NADPH that power the next stage.
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