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Linear Electron Flow (PS I) • In PS I (like PS II) Ele Primary acceptor 1/ 2 H+ + O2 2 H2 O e– 2 ctr 4 tra nsp on Pq cha Cytochrome complex 3 transferred light • Primary acceptor ort energy excites P700 e– in • loses an electron Pc e– e– to acceptor P700 5 P680 Light 1 Light 6 • P700+ ATP Pigment molecules accepts electron • Photosystem I (PS I) Photosystem II (PS II) from PS II • via the electron transport chain Linear Electron Flow (PS I) • Ele Primary acceptor 2 H+ + 1/ O 2 2 H2 O e– 2 ctr on Pq 4 tra ns Primary acceptor port cha in e– Cytochrome complex 3 Each electron • E tra lect n ro ch spo n ain rt • Fd e– – e 8 NADP + + H+ NADP + reductase NADPH Pc e– e– falls down electron transport chain 7 • P700 5 P680 Light 1 Light from primary electron acceptor of PS I 6 to ferredoxin (Fd) ATP • Pigment molecules electrons then transferred to NADP+ Photosystem I (PS I) Photosystem II (PS II) • reduced to NADPH • Cyclic Electron Flow • Cyclic electron flow • uses only photosystem I and produces ATP • but not NADPH • no oxygen released • generates surplus ATP • meeKng demand for Calvin cycle • Some organisms such as purple sulfur bacteria • have PS I but not PS II • thought to have evolved before linear electron flow • may protect cells from light‐induced damage goes to Calvin cycle Fig. 10‐15 Primary acceptor Primary acceptor Fd Fd Pq NADP+ reductase Cytochrome complex NADP+ + H+ NADPH Pc Photosystem I ATP Photosystem II What Makes the Light ReacKons So Important? • Two acKons of great consequence take place in the light reacKons 1. Water is split, yielding both electrons and oxygen. • The electrons move through the light reacKons. • The oxygen is what organisms such as ourselves breathe in. 2. The electrons that are derived from the water • and then given an energy boost by the sun’s rays • are transferred to a different molecule: the iniKal electron acceptor • This is the means through which the sun’s energy is transferred into the living world Importance of Light ReacKons • Also… • fall of electrons through the electron transport chain between photosystems II and I • yields energy that produces ATP • used to power the second stage of photosynthesis Light ReacKons PLAY Light Reactions STROMA (low H+ concentration) Cytochrome Photosystem I complex Light Photosystem II 4 H+ Light Fd NADP+ reductase H 2O THYLAKOID SPACE (high H+ concentration) e– 1 e– 1/ NADP+ + H+ Pc 2 2 3 NADPH Pq O2 +2 H+ 4 H+ To Calvin Cycle Thylakoid membrane ATP synthase STROMA (low H+ concentration) ADP + Pi ATP H+ The Calvin Cycle • Carbon dioxide from the atmosphere • Combined with a sugar • RuBP (Ribulose‐1,5‐bisphosphate) • RuBisCo (Ribulose‐1,5‐bisphosphate carboxylase/oxygenase) • Enzyme that catalyzes RuBP and CO2 • resulKng compound • energized with addiKon of electrons • Makes 3‐carbon sugars • supplied by light reacKons • Later combined into carbohydrates The Calvin Cycle • G3P • Glyceraldehyde 3‐phosphate • High‐energy sugar • The result of the Calvin cycle • Final product of photosynthesis • Can be used for energy or plant structure The Calvin Cycle Input 3 CO2 (Entering one at a time) Phase 1: Carbon fixation Rubisco 3 P Short-lived intermediate P 6 P 3-Phosphoglycerate 3P P Ribulose bisphosphate (RuBP) 6 ATP 6 ADP 3 ADP 3 Calvin Cycle 6 P P 1,3-Bisphosphoglycerate ATP 6 NADPH Phase 3: Regeneration of the CO2 acceptor (RuBP) 6 NADP+ 6 P i 5 P G3P 6 P Glyceraldehyde-3-phosphate (G3P) 1 Output P G3P (a sugar) Phase 2: Reduction Glucose and other organic compounds The Calvin Cycle • All these steps are powered by ATP produced in the light reacKons Summary PhotorespiraKon and the C4 Pathway • In plants the enzyme rubisco frequently binds with oxygen rather than with carbon dioxide • • a process called photorespiraKon that undercuts photosynthesis. PhotorespiraKon • problem increases as temperature rises • because as plants close their stomata to keep in moisture • they also keep out CO 2 • increasing likelihood that rubisco will bind with oxygen • Rubisco‐oxygen then metabolized • EvoluKonary relic? • Releasing carbon dioxide • Atmospheric O2 low when rubisco evolved • May be protecKve • Against damage from buildup of light rxn products PhotorespiraKon • Some warm‐climate plants • Have evolved a means of dealing with photorespiraKon • C4 photosynthesis • Most plants C3 C4 Photosynthesis • PEP carboxylase • enzyme that binds with carbon dioxide but not with oxygen • Forms a 4‐carbon compound (Hence C4) • OxaloaceKc acid • From combinaKon of CO2 with a 3‐carbon molecule phosphoenolpyruvate (PEP) • Occurs in the mesophyll cells • OxaloaceKc acid • then shucled to bundle sheath cells to Calvin cycle • where CO2 is released to bind with rubisco C4 Photosynthesis The C4 pathway C4 leaf anatomy Mesophyll cell Mesophyll cell CO2 PEP carboxylase Photosynthetic cells of C4 Bundleplant leaf sheath cell Oxaloacetate (4C) PEP (3C) ADP Vein (vascular tissue) Malate (4C) Stoma Bundlesheath cell ATP Pyruvate (3C) CO2 Calvin Cycle Sugar Vascular tissue CAM Plants • Crassulacean Acid Metabolism • Dry‐weather plants such as cacK (Crassulacea) employ another form of photosynthesis • CAM photosynthesis CAM Photosynthesis • CAM photosynthesis • stomata open only at night • Carbon dioxide is “banked” unKl sunrise • ledng in and fixing carbon dioxide • when photons supply energy for Calvin cycle. Photosynthesis Photosynthesis Song You should now be able to: 1. Describe the structure of a chloroplast 2. Describe the relaKonship between an acKon spectrum and an absorpKon spectrum 3. Trace the movement of electrons in linear electron flow 4. Trace the movement of electrons in cyclic electron flow 5. Describe the similariKes and differences between oxidaKve phosphorylaKon in mitochondria and photophosphorylaKon in chloroplasts 6. Describe the role of ATP and NADPH in the Calvin cycle 7. Describe the major consequences of photorespiraKon 8. Describe two important photosyntheKc adaptaKons that minimize photorespiraKon 5. Describe the similariKes and differences between oxidaKve phosphorylaKon in mitochondria and photophosphorylaKon in chloroplasts 6. Describe the role of ATP and NADPH in the Calvin cycle 7. Describe the major consequences of photorespiraKon 8. Describe two important photosyntheKc adaptaKons that minimize photorespiraKon