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 – 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)

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

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

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 O 2 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+  NADPH – NADPH transferred to stroma to the Calvin Cycle H+ from inner thylakoid membrane passed through ATP Synthase to make ATP – ADP + Pi  ATP