Light Reaction (Photochemical Phase)
Light reactions involve light absorption, water splitting, oxygen release, and the formation of ATP and NADPH (high-energy chemical intermediates).
The Electron Transport
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When PS II absorbs red light of 680 nm, electrons are excited and transferred to an electron acceptor.
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The electron acceptor passes them to a chain of the electron transport system consisting of cytochromes.
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This movement is downhill in terms of redox potential scale.
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The electrons are then transferred to the pigments of PS I.
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Simultaneously, electrons in PS I are excited by red light of 700 nm and transferred to another acceptor molecule with a greater redox potential.
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These electrons move downhill to NADP⁺, reducing it to NADPH + H⁺.
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The transfer of electrons from PS II to PS I and finally to NADP⁺ is called the Z scheme, named for its zigzag shape on a redox potential scale.
Splitting of Water (Photolysis)
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The water-splitting complex in PS II is located on the inner side of the thylakoid membrane.
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PS II continuously supplies electrons by replacing those from water splitting, providing electrons to replace those removed from PS I.
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The protons (H⁺) are used to reduce NADP⁺ to NADPH.
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Oxygen is released as a by-product of photosynthesis.
2H₂O → 4H⁺ + O₂ + 4e⁻
Photophosphorylation
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Phosphorylation is the synthesis of ATP by cells in mitochondria and chloroplasts.
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Photophosphorylation is ATP synthesis from ADP in chloroplasts in the presence of light, occurring in two ways: non-cyclic and cyclic.
a) Non-Cyclic Photophosphorylation
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Occurs when PS II and PS I work in series through an electron transport chain, as in the Z scheme.
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Both ATP and NADPH + H⁺ are synthesized.
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It is non-cyclic because electrons lost by PS II do not return to it but pass to NADP⁺.
b) Cyclic Photophosphorylation
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Occurs in stroma lamellae when only PS I is functional.
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The electron circulates within the photosystem, and ATP synthesis occurs due to the cyclic flow of electrons.
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Grana lamellae have both PS I and PS II, but stroma lamellae lack PS II and NADP reductase.
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Electrons do not pass to NADP⁺ but cycle back to PS I through the electron transport chain.
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Only ATP is synthesized (no NADPH + H⁺).
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Cyclic photophosphorylation occurs when only light wavelengths beyond 680 nm are available.
Chemiosmotic Hypothesis
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The chemiosmotic hypothesis explains the mechanism of ATP synthesis in chloroplasts.
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Chemiosmosis is the movement of ions across a semipermeable membrane, occurring in chloroplasts and mitochondria.
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It requires a membrane, a proton pump, a proton gradient across thylakoid membranes, and ATP synthase.
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Water splitting on the inner side of the membrane causes protons to accumulate in the thylakoid lumen.
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Protons are transported across the membrane as electrons move through photosystems, due to:
- The primary electron acceptor, located on the outer side of the membrane, transfers electrons to an H carrier, removing a proton from the stroma. When the electron is passed to an electron carrier on the inner side, a proton is released into the lumen.
- The NADP reductase enzyme, on the stroma side, requires protons from the stroma along with electrons from PS I to reduce NADP⁺.
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This decreases protons in the stroma and accumulates them in the lumen, creating a proton gradient and lowering the pH in the lumen.
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The breakdown of the proton gradient drives ATP synthesis via the ATP synthase enzyme, which consists of:
- CF₀: Embedded in the membrane, forming a transmembrane channel for facilitated diffusion of protons to the stroma, breaking down the proton gradient.
- CF₁: Protrudes on the outer surface of the thylakoid membrane, where the energy from the gradient breakdown causes a conformational change, enabling ATP synthesis.
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Energy from proton pumping creates a high proton concentration in the thylakoid lumen. ATP synthase channels protons back across the membrane, releasing energy to catalyze ATP formation.