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In photosynthetic light reactions, chlorophyll absorbs photons and uses the energy to split water, releasing O₂ and supplying electrons that travel through the electron transport chain to generate NADPH and ATP. These energy carriers fuel the Calvin cycle, where CO₂ is fixed into carbohydrates.
Green plants rely on chlorophyll to capture light energy. When a photon hits a chlorophyll molecule, one of its electrons is excited to a higher energy state. This energized electron is transferred to the primary electron acceptor, initiating the electron transport chain that culminates in the reduction of NADP⁺ to NADPH.
To replenish the lost electron, chlorophyll oxidizes water molecules in a process known as photolysis. Each water molecule splits into two oxygen atoms, which combine to form O₂ gas released into the atmosphere, and two protons (H⁺) plus two electrons. The protons enter the thylakoid lumen, creating a proton gradient that powers ATP synthesis via ATP synthase.
Thus, water is the sole electron donor in the light reactions, and its oxidation supplies both the electrons required for NADPH formation and the protons that drive ATP production.
With NADPH and ATP generated, the plant enters the Calvin cycle, the set of dark reactions that fix atmospheric CO₂ into sugars. NADPH donates electrons to reduce ribulose‑1,5‑bisphosphate (RuBP), while ATP provides the energy for the catalytic steps. The net result is the synthesis of a carbohydrate with the empirical formula CH₂O, most commonly glucose (C₆H₁₂O₆).
While the Calvin cycle proceeds without light, it is tightly coupled to the light reactions because it depends on the energy carriers produced there. Oxygen released during photolysis is simultaneously taken up by the plant for cellular respiration, completing the energy cycle.
