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Photosynthesis Overview and Chloroplast Ultrastructure

Stay updated with the latest class announcements regarding homework submissions, readings, and review sessions for the upcoming exam, covering topics like photosynthesis, chloroplast ultrastructure, and intracellular trafficking.

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Photosynthesis Overview and Chloroplast Ultrastructure

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  1. Announcements, March 12 • Please remove homework from folders • One homework and 2 quizzes (Q14, Q18) without names still unclaimed – please see me to claim. • Reading for today: pp. 288-302 on photosynthesis; pp. 302-307 will not be covered due to snow day. • Review Session from 4-5 PM, March 13 in BR 203 • Exam 2 on Wednesday, March 14 over the same material as before. • Reading for Friday, March 16: pp. 318-358 on intracellular trafficking

  2. Chloroplast Ultrastructure

  3. Photosynthesis Overview • Light + 3 CO2 + 6 H2O → C3H6O3 + 3 O2 + 3 H2O • Function: ATP synthesis and synthetic rxns • Energy source: sunlight • Energy conversion: • solar  H+ gradient  ATP • H2O  electron transport  NADPH • Carbon source: CO2 • Electron source: H2O • Intermediate electron acceptor: NADP+ • Byproduct: O2 Reduced C: 3C sugar

  4. Energy Transduction reactions: Photoreduction (NADPH synthesis) Photophosphorylation (ATP synthesis)

  5. NADP+ and NADPH Carry e- in synthetic reactions, whereas NAD+/NADP used in degradative rxns.

  6. Absorption spectra of plant pigments

  7. Chlorophyll Chlorophyll is special because it can accept a low energy electron and donate a high energy electron.

  8. Light Harvesting Complex 1. Heat, light 2. 3.

  9. Fate of light energy when e*  eo • Energy is lost as heat and lower energy light. • Resonance energy transfer to an electron in an adjacent pigment molecule. • Photochemical reduction, where e* is itself transferred to a carrier (only in reaction center chlorophyll).

  10. Resonance Energy Transfer to Adjacent Pigments Excited state e* eo e* eo e* eo e* eo photon Ground state Reaction center chlorophyll P680 PSII Antenna chlorophylls

  11. Photochemical Reduction:e* is transferred to NADP+ e* eo NADP+  NADPH photon OEC ETS e* eo e* e* e e* H2O  2H+ + O2 Reaction center chlorophyll P680 PSII Electron transport System (ETS) Chlorophyll P700 PSI

  12. Energy conversion in thylakoid MB Analogous To RCIII

  13. The Z Scheme: Steps 1. Water splitting: 2 H2O → 4 H+, O2 to lumen 2. P680o P680* (PSII) 3. Electron transfer to P700o (PSI) 4. 4 H+ pumped to lumen through cytochrome b6/f 5. P700o  P700* (PSI) 6. Electron transfer to ferredoxin 7. Ferredoxin-NADP+ reductase catalyzes: 2 NADP+ + 2 H+stroma + 2 e-  2 NADPH

  14. Understanding how the water-splitting enzyme works is a major unsolved problem, with practical applications 4

  15. Z scheme

  16. H+ Cl- lumen stroma lumen stroma H+ H+ Mg2+ Photophosphorylation(Light-driven ATP synthesis) Current estimates (know this): • pHlumen = 6 • pHstroma = 8 • pH = -2 (in-out) • Em = +30 mV (flow of Mg2+ and Cl- balances much of flow of H+) • pmf = 30 - 60 mV(-2) = +150 mV • 4 H+ flow/ATP through CF0CF1 ATP synthase • 4 e- Z scheme  2 ATP, 2 NADPH • But need 3 ATP/2 NADPH in Calvin cycle, so cyclic electron flow occurs:

  17. Cyclic e- Flow • Regulates NADPH/ATP formed (need 2:3). • 2 electrons flow back through cyt b6/f to P700, pumping 4 H+ into lumen • 2 photons re-excite the 2 electrons. • No water oxidized and no O2 released, since PSII not involved. NADP+ PSI

  18. Photosynthesis Reactions(Don’t memorize) 1. Energy transduction reactions (Equation 11-14) 26 photons + 9 ADP + 9 Pi + 6 NADP+ + 6 H2O  3 O2 + 9 ATP + 6 NADPH + 3 H2O 2. Calvin cycle reactions (Equation 11-13) 3 CO2 + 9 ATP + 6 NADPH + 6 H2O  C3H6O6 + 6 NADP+ + 9 ADP + 9 Pi 3. Overall (Equation 11-16): 26 photons + 3 CO2 + 6 H2O  C3H6O6 + 3 O2 + 3 H2O What % of mass of C3H6O6 comes from CO2 ? MW of CO2 is 44, of H2O is 18, of C3H6O6is 186.

  19. Carbon fixation occurs in the stroma

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