The reactions of photosynthesis that use energy from light to produce ATP and NADPH are known as the light-dependent reactions or the photochemical phase of photosynthesis. These occur in the thylakoid membranes of chloroplasts and can be broken down into several key steps:
Key Steps of Light-Dependent Reactions:
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Photon Absorption: Chlorophyll and other pigments absorb light energy, primarily at the blue and red wavelengths. This energy excites electrons in the chlorophyll molecules.
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Water Splitting (Photolysis): The absorbed light energy drives the splitting of water molecules (H₂O) into oxygen (O₂), protons (H⁺), and electrons (e⁻). This reaction is catalyzed by the enzyme photosystem II (PSII). \[ 2H₂O \rightarrow 4H⁺ + 4e^- + O₂ \]
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Electron Transport Chain (ETC): The excited electrons from chlorophyll are transferred to the electron transport chain. As the electrons move through a series of proteins (including plastoquinone, cytochrome b6f, and plastocyanin), they release energy that is used to pump protons into the thylakoid lumen, creating a proton gradient.
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ATP Synthesis: The proton gradient created across the thylakoid membrane drives ATP synthesis through ATP synthase (a process known as chemiosmosis). As protons flow back into the stroma through ATP synthase, ATP is produced from ADP and inorganic phosphate (Pi). \[ ADP + Pi \rightarrow ATP \]
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NADPH Formation: The electrons that move through the ETC eventually reach photosystem I (PSI). Here, they are re-excited by photons and ultimately used to reduce NADP⁺ (with the help of the enzyme NADP+ reductase) to form NADPH. \[ NADP^+ + 2e^- + H^+ \rightarrow NADPH \]
Summary of Products:
- ATP: Provides energy for the subsequent light-independent reactions (Calvin cycle).
- NADPH: Serves as a reducing agent, providing the necessary electrons for the reduction of carbon dioxide in the Calvin cycle.
Overall Equation for Light-Dependent Reactions:
The overall reaction that summarizes the light-dependent processes can be represented as: \[ 2H₂O + light \rightarrow O₂ + ATP + NADPH \]
These products (ATP and NADPH) are essential for the subsequent light-independent reactions, where carbon fixation occurs to ultimately produce glucose and other carbohydrates.