Generating Chemical Energy Brought to you by Photosynthesis

(INTERIOR OF THYLAKOID) Photosystems In the thylakoid membrane Composed of Reaction-center complex Proteins w/ chlorophyll a Light-harvesting complex Protein w/ chlorophyll a, b and carotenoids act as light-gathering “antenna complex” Primary election acceptor Photon Thylakoid Light-harvesting complexes Reaction center Photosystem STROMA Thylakoid membrane Transfer of energy Special chlorophyll a molecules Pigment THYLAKOID SPACE (INTERIOR OF THYLAKOID) Figure 10.12 e–

Photosystems Light-harvesting complex direct NRG of photons to reaction center Reaction-center absorbs energy: Special chlorophyll a molecule donates e- instead of letting it fall back to ground state. e- gets bumped up to a primary electron acceptor REDOX This is first step of light reactions

Different types Photosystems I and II PSII PSI Named by discovery not sequence PSII works first then PSI PSII reaction center chlorophyll a absorbs light @ 680 nm. PSI reaction center chlorophyll a absorbs light @ 700nm Work together to generate ATP and NADPH 6 steps

Chemical NRG Production Step 1 Photon excites p680 chlorophyll and donates e- to primary electron acceptor REDOX P680  p680+ + e- Problem can this process occur twice? Why?

Chemical NRG Production Step 2 H2O split H2O  2H+ + 2e- + O 2H+ donated to thylakoid space for ETC Oxygen kicked out And … Answer to original problem e- fed to oxidized p680+ chlorophyll Reduces back to p680, ready to do it again

Chemical NRG Production Step 3 Excited e- passed from primary acceptor to PS I by ETC

Electron Transport Chain Use gradient of H+ ions to rotate ATP synthesis and crush ADP to Pi (inorganic phosphate) https://www.youtube.com/watch?v=PjdPTY1wHdQ

Chemical NRG Production Step 4 Energetically “falling” e- supply NRG to H+ pump Electrochemical gradient produced Chemiosmosis generates ATP by synthase protein

Chemical NRG Production Step 5 PS I looses e- to primary acceptor by photon excitement (same as step 1 but in PS I) e- from ETC reaches PS I and donates to now oxidized p700+

Chemical NRG Production Step 6 Excited e- passed to 2nd ETC No H+ pumps just REDOX Passed to NADP+ reductase and reduces NADP+ to NADPH (NADP+ + 2e-  NADPH

Figure 10.14 NADPH e– Mill makes ATP Photon Photosystem II

Still with me?? Light is used to generate NADPH and ATP Key to this is the flow of excited electrons Redox reactions!! 2 possible routes Cyclic Noncyclic Usually used

Cyclic / linear flow Produces NADPH, ATP, and oxygen Photosystem II (P680) to primary acceptor Through an E-T.C. to Photosystem I ATP produced Noncyclic photophosphorylation P700 to primary acceptor Through 2nd E-T.C. to NADP+ NADPH

Figure 10.14 NADPH e– Mill makes ATP Photon Photosystem II

Cyclic Electron Flow Photosystem I used, not Photosystem II Chlorophyll a is recycled back down to its ground state Photon hits P700 (PS I), excites e- passing it to primary electron receptor e- don’t go to NADP+ reductase Enter into electron chain return to P700 to complete the cycle Produces ATP, not O2 or NADPH

Primary Fd acceptor Pq NADP+ reductase Cytochrome NADPH complex Pc ATP Figure 10.15 Photosystem II Photosystem I

Why is a Cyclic form needed? Noncyclic e- flow produces ATP and NADPH in = equal quantities Calvin Cycle uses more ATP than NADPH Does it make sense now? Cyclic electron flow makes ATP only The concentration of NADPH regulate path Reduce all NADP+ non cyclic flow shuts down No where for e- to flow but back to ETC

Quick Summary: Goal: Light reactions use _________ power to generate _________ and _______ which provide chemical energy and reducing power to the Calvin Cycle. The Calvin Cycle makes _____________. Noncyclic photophosphorylation Photosystem(s) used: What are the products: Cyclic photophosphorylation

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A Comparison of Chemiosmosis in Chloroplasts and Mitochondria Chemiosomosis: An nrg coupling mechanism that uses nrg stored in a H+ gradient across a membrane to drive cellular work (ATP synthesis) Chloroplasts and mitochondria generate ATP this way In both organelles, an electron transport chain pumps protons across a membrane as electrons are passed along a series of increasingly electronegative carriers. This transforms redox energy to a proton-motive force in the form of an H+ gradient across the membrane. ATP synthase molecules harness the proton-motive force to generate ATP as H+ diffuses back across the membrane.

Difference: What form does the energy come from in: Cellular Respiration? Photosynthesis Key Higher [H+] Lower [H+] Mitochondrion Chloroplast MITOCHONDRION STRUCTURE Intermembrance space Membrance Matrix Electron transport chain H+ Diffusion Thylakoid Stroma ATP P ADP+ Synthase CHLOROPLAST Figure 10.16