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Lecture 6 - Photosynthesis: The Light Reactions Chem 454: Regulatory Mechanisms in Biochemistry University of Wisconsin-Eau Claire Introduction Photosynthesis represents for the biosphere the major source of Free energy (1017 kcal/year stored) Carbon (1010 tons/year assimilated) Oxygen 2 Overview Analogous to the combination of oxidative phosphorylation and the citric acid cycle. 3 1. Chloroplasts Like oxidative phosphorylation, photosynthesis is compartmentalized 4 1 Mitochondria Mitochondria are bound by a double membrane 5 2. Electron Transfer Light absorption by chlorophyll induces electron transfer 6 2. Electron Transfer Absorption of light leads to photoinduced charge separation 7 2. Electron Transfer Absorption of light leads to photoinduced charge separation 8 2.1 Bacteria vs. Green Plants Plants have two photosystems Photosystem I 13 polypeptides chains 60 Chlorophylls 3 4Fe-4S centers 1 Quinone Photosystem II 10 polypeptide chains 30 Chlorophylls 1 Non-heme Fe 4 Mn+2 (Mn+2, Mn+3, Mn+4 , Mn+5) 9 2.1 Bacteria vs. Green Plants Bacteria have a single system 4 Bacteriochlorophylls b 2 Bacteriopheophytin b 2 Quinones 1 Fe2+ 10 2.1 Bacteria vs. Green Plants Bacteria have a single system 11 2.2 The Bacterial Photosynthetic Reaction Center 12 2.3 Electron Transfer The rate of electron transfer is dependent up two factors: Distance 13 Driving Force 3.3 Q-Cytochrome c Oxidoreductase The Q cycle: 14 3. Two Photosystems of Plants In green plants there are two photosynthetic reaction centers. 15 3.1 Photosystem II The core of PSII is similar to the bacterial system. 16 3.1 Photosystem II 17 3.1 Photosystem II Four photons are required to generate one oxygen molecule 18 3.1 Photosystem II The 4 Manganese center is where the electrons are extracted from the water. 19 3.1 Photosystem II An equivalent of 4 H+ are moved across the thylakoid membrane by PSII 20 3.2 Cytochrome bf The cytochrome bf complex transfers the electrons from plastoquinone to platocyanin: 21 3.2 Cytochrome bf 22 3.3 Q-Cytochrome c Oxidoreductase The Q cycle: 23 3.3 Photosystem I Though larger, the core of PSI is also similar to the bacterial system. 24 3.3 Photosystem I The receptor of the the electrons from PSI is the small one-electron redox protein, Ferredoxin. 25 3.3 Photosystem I 26 3.3 Photosystem I The Z-scheme of photosynthesis 27 3.4 + Ferredoxin-NADP + Ferredoxin-NADP reductase 28 Reductase 3.4 + Ferredoxin-NADP + Ferredoxin-NADP coenzyme FAD. 29 Reductase reductase contains the 3.4 + Ferredoxin-NADP + Ferredoxin-NADP coenzyme FAD. 30 Reductase reductase contains the 4. The Proton Gradient + ferredoxin-NADP PSII, Cytochrome bf and reductase each contribute to the proton gradient. 31 4. The Proton Gradient André Jagendorf's demostration 1966, was one of the earliest pieces of evidence to support Peter Mitchell's chemiosmotic hypothesis. 32 4.1 ATP Synthase The ATP synthase closely resembles the ATP synthase of mitochondria 33 4.1 ATP Synthase Comparing photosynthesis to oxidative phosphorylation: 34 4.2 Flow of Electrons Through Photosystem I Normally photosynthesis produces both ATP and NADPH. + NADP When there is not available, cyclic flow of electrons through PSI can be used + to produce ATP without reducing NADP 35 4.2 Flow of Electrons Through Photosystem I Cyclic flow of electrons through PSI: 36 4.3 Stoichiometry of Photosynthesis 12 protons are expected to be used for one turn of the ATP synthase, therefore, producing 3 AtP's from 3ADP's and 3Pi's 37