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Photosynthesis The Light Reaction

Photosynthesis The Light Reaction. Photosynthesis takes place in the chloroplasts in areas known as photosystems Photosystems - complexes containing the pigments where light energy is harvested Chlorophyll is the main pigment in the photosystems of eukaryotic plants

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Photosynthesis The Light Reaction

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  1. PhotosynthesisThe Light Reaction • Photosynthesis takes place in the chloroplasts in areas known as photosystems • Photosystems - complexes containing the pigments where light energy is harvested • Chlorophyll is the main pigment in the photosystems of eukaryotic plants • The light reaction takes place in the thylakoid membranes • The thylakoid membranes are populated by two types of photosystems (PS I & PS II). The two work together to produce ATP & NADPH (8 steps in the non-cyclic flow of electrons)

  2. Photosystem II • When a photon is absorbed by a pigment it causes an e- to move from its ground state to an excited state. • The electron (because it is unstable in its new configuration) immediately returns to its ground state releasing the stored energy. • photon energy is transferred from pigment to pigment (light harvesting complex) until it reaches the reaction center • The energy is then harvested in a reaction center where it is transferred to an electron acceptor. • Water is split • 2 H+ are produced • Moved into the inner thylakoid space where they provide the energy to produce ATP • 2 e- are given back to the P680 • O2 is produced from the split water • The excited e- is passed through an electron transport chain (ETC) to PS I • The exergonic "fall" of electrons provides energy for the synthesis of ATP along with the proton (H+) motive force created by the movement of H+ across the membrane

  3. Photosystem I • PS I absorbs a photon and donates an e- • 2 electrons from PS II fill in the space (electron hole) • Electrons captured by the reaction center are transferred down a 2nd ETC • Enzyme NADP+ reductase transfers the 2 e- to NADP+ to NADPH (requires 2 e-) • NADP+ is an electron carrier • NADPH is then transferred to the Calvin Cycle where it will be used to produce glucose

  4. Summary • Step 1 • Light absorbed by PS II splits water • 2 e- to ETC to PS I • H+ moved to create gradient to form ATP • O2 released • Step 2 • Light absorbed by PS I reenergizes e- from PS II • Energized e- passed to ETC & is used to produce NADPH • 2e- + NAD+ g NADPH • NADPH transferred to stroma to the Calvin Cycle • H+ from inner thylakoid membrane passed through ATP Synthase to make ATP • ADP + Pi g ATP

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