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 on photosynthesis; pp 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 on intracellular trafficking

Chloroplast Ultrastructure

Photosynthesis Overview Light + 3 CO H 2 O → C 3 H 6 O O H 2 O Function: ATP synthesis and synthetic rxns Energy source: sunlight Energy conversion: –solar  H + gradient  ATP –H 2 O  electron transport  NADPH Carbon source: CO 2 Electron source: H 2 O Intermediate electron acceptor: NADP + Byproduct: O 2 Reduced C: 3C sugar

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

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

Absorption spectra of plant pigments

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

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

Fate of light energy when e*  e o 1.Energy is lost as heat and lower energy light. 2.Resonance energy transfer to an electron in an adjacent pigment molecule. 3.Photochemical reduction, where e* is itself transferred to a carrier (only in reaction center chlorophyll).

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

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

Energy conversion in thylakoid MB Analogous To RCIII

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

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

Solving how water splitting works could lead to regenerative fuel cells Regenerative Fuel Cells. Regenerative fuel cells would be attractive as a closed-loop form of power generation. Water is separated into hydrogen and oxygen by a solar- powered electrolyser. The hydrogen and oxygen are fed into the fuel cell which generates electricity, heat and water. The water is then recirculated back to the solar- powered electrolyser and the process begins again. These types of fuel cells are currently being researched by NASA and others worldwide. -- From H2OH2O

Z scheme

Photophosphorylation (Light-driven ATP synthesis) Current estimates (know this): pH lumen = 6 pH stroma = 8  pH = -2 (in-out) E m = +30 mV (flow of Mg 2+ and Cl - balances much of flow of H + ) pmf = mV(-2) = +150 mV 4 H + flow/ATP through CF 0 CF 1 ATP synthase 4 e -  Z scheme  2 ATP, 2 NADPH But need 3 ATP/2 NADPH in Calvin cycle, so cyclic electron flow occurs: H + Cl - H + H + Mg 2+ lumen stroma lumen stroma

Cyclic e - Flow Regulates NADPH/ATP formed (need 2:3). 2 electrons flow back through cyt b 6 /f to P700, pumping 4 H + into lumen 2 photons re-excite the 2 electrons. No water oxidized and no O 2 released, since PSII not involved. NADP + PSI

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

Carbon fixation occurs in the stroma