1Overview of photosynthesis1. Light absorptionwater is splitÆ O22. NADPH Reducing energy is formed3. ATPChemical energy is formed4. CO2-->--> sugarCO2is converted to sugarsQ? How are electrons from water transferred to NADPH?How is ATP formed?Photosynthesis is a light-driven redox process.1. The entire process occurs in several steps, because there is insufficient energy to boost e- from H2O directly into NADP+..H2O + NADP++ ADP + Pi --->1/2 O2+ NADPH + H++ ATPO2evolving organisms have 2 photosystems that operate in series.2. Organisms obtain energy from ox-red reactions.3. Plants use 2 photosystems. Each PS has a different function.A) PSII: pull electrons from waterb) PSI: e- reduce NADP+ to form reducing power4. Two reaction centers are connected by an electron transport chain5. Electron transport and water splitting generates a proton gradient that is used to make ATP.7-34b. Oxygen evolving organisms have 2 photosystems in series.7-21 Taiz. Z scheme. Electron transport carriers at their redox potentials. Energy is needed to boost e- from water to NADPH.Organisms obtain energy from oxidation/ reduction reactions1. Tendency for a redox reaction to proceed depends on the difference in energy of the transferable electron in the two molecules.The reducing potential is a measure of the readiness with which an atom takes up an electron. Measured as a voltage.If reducing potential is -, I.e. lower affinity for electron than H2/2H+∆ E = E (acceptor) – E (donor) Table. Mid point redox (reducing) potentials of selected redoxcouples from respiration and photosynthesis.-----------------------------------------------Em (V)----------------------------------------------Ferredoxin ox/red -0.422H+/NAD+/ NADH -0.32NADP++2H+/ NADPH + H+-0.322H+/H20Ubiquinone +0.040Cyt c ox/cyt c red +0.220S + 2H+ /H2S +0.231/2O2+ 2H+/ H2O +0.82-------------------------------------------------Useful equations to analyze energy changes in redox reactions:∆E = E (acceptor) - E (donor) E = reducing potential (V)∆G = - zF∆E ∆E = difference in the reducing potential (v)z = number of electrons transferredF = Faraday's constant, 23 kcal/V.mol27-23. How do photosystems work?In the excited state, electron can be lost from chl a.7-14 Taiz. Two photosystems operate in series. PSII Reaction center chla = 680PSI has a RC pigment chla, P7007-22. Two Light reactions are connected by several electron transport carriers7-24. PS II. A large ComplexPS I37-24 Taiz. Plastoquinone is a mobile 2-electron carrier7-22. TaizHow is ATP formed?Electron transport and water splitting form a proton motive force (pmf)PMF is used to make ATPWhere are protons produced?1. Splitting of water2. PQ oxidation7-32 Taiz: How is ATP formed?Expt showing energy in pH gradient is converted to ATP in the dark.pH 4pH 8thylakoidBox 7-3 Taiz7-317-11 Taiz -Why is there a red drop?In lab.Why did you add Ferric