photosynthesis. the light reactions occur on the thylakoid membranes of chloroplasts (or the plasma...
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PHOTOSYNTHESIS
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Compare photosynthesis to respiration: Respiration: C6H12O6 + 6O2 6CO2 + 6H2O
Go’ = -2840 kJ/mol Photosynthesis in green plants: 6CO2 + 6H2O C6H12O6 + 6O2
Go’ = +2840 kJ/mol (Unfavorable Go’ explains the need for energy input.)
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Compare photosynthesis in plants with bacteria: Plants (divided by 6, H2O added to each side) CO2 + 2 H2O C(H2O) + O2 + H2O Photosynthesis in green sulfur bacteria CO2 + 2H2S C(H2O) + 2S + H2O Photosynthesis in purple non-sulfur bacteria CO2 + 2CH3CHOHCH3 C(H2O) + 2CH3COCH3+ H2O isopropanol acetone CO2 + 2 CH3CHOHCOOH C(H2O) + 2 CH3COHCOOH + H2O lactate pyruvate Generalize: CO2 + 2H2D C(H2O) + 2D + H2O D = H/electron donor
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Compare photosynthesis in plants with bacteria: Plants (divided by 6, H2O added to each side) CO2 + 2 H2O C(H2O) + O2 + H2O Photosynthesis in green sulfur bacteria CO2 + 2H2S C(H2O) + 2S + H2O Photosynthesis in purple non-sulfur bacteria CO2 + 2CH3CHOHCH3 C(H2O) + 2CH3COCH3+ H2O isopropanol acetone CO2 + 2 CH3CHOHCOOH C(H2O) + 2 CH3COHCOOH + H2O lactate pyruvate Generalize: CO2 + 2H2D C(H2O) + 2D + H2O D = H/electron donor
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CO2 + 2H2D C(H2O) + 2D + H2O D = H/electron donor CO2 dark C(H2O) + H2O 2 NADPH + H+ 2 NADP+ Note the reduction of C! 3 ATP 3 ADP + 3Pi 2D light 2H2D So the light reactions must produce NADPH and ATP to provide energy and H
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The light reactions occur on the thylakoid membranes of chloroplasts (or the plasma membrane or internal membranes of bacteria).
(Note the “lumen” of thylakoids)
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The light reactions occur on the thylakoid membranes of chloroplasts (or the plasma membrane or internal membranes of bacteria).
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Light reactions
• 2 photosystems • transfer of e- from H2 ( O in th e lum )en t o
NADP+ ( in the strom )a • transfer of H+ from stroma to lumen (and ba ck
through ATP syntha )se
(“non-cyclic photophosphorylation”)
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Light reactions
• 2 photosystems • transfer of e- from H2 (O lumen) to NADP+
(strom )a • transfer of H+ from stroma to lumen (and ba ck
through ATP syntha )se
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Why two photosystems? • : PSI strong reducta ,ntweak oxidant • : PSII weak reduct , ant strong oxidant
(Bacterial systems, except cyanobacterialsystems, have only one photosystem.)
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Cyclic photophosphorylation
• Photosystem I only • transfer of e- from PSI to cyt b/f and back • transfer of H+ from stroma to lumen (and ba ck
through ATP syntha )se
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Cyclic photophosphorylation
• Photosystem I only • transfer of e- from PSI to cyt b/f and back • transfer of H+ from stroma to lumen (and ba ck
through ATP syntha )se
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There is a partial separation of PS I and PSII in thylakoids.
•More PSI and ATP synthase in stroma lamellae•More PSII in grana interiors
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Reduction of CO2 – “Calvin-Benson cycle”
♦ Soluble enzyme in strom a(“Rubis ”) co adds CO2 t o
( RuBP as )ugar ♦ NADPH adds electrons ♦ Free energ y of NADPH
oxidation and ATP hydrolysi spush the reaction forward
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Carboxylation: • ribulose bis phosphate carboxylase-oxygena : se
Rubisco CO2 + RuBP + H2 O (6 )C 2 3-PGA 6 CO2 + 6RuBP + 6H2 O 12 3-PGA Reduction:
• P-glycerate kina se and G-3-P dehydrogenase 3- + PGA ATP 1,3-BPG + ADP
1,3 - + BPG NADPH G-3-P + NAD+ + Pi
12 3- +PGA 12 ATP + 12 NADPH + 12 H+ 12 G-3- + P 12 ADP + 12Pi + 12NADP+ Regeneration of RuBP
10 G-3- +P 6 ATP 6 RuBP + 6 ADP 2 G-3- P glucose through reverse glycolysis (see next lecture)
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Regeneration of RuBP
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Convert 10 G-3-P into 6 RuBP
4 G-3-P 2 F-6-P 2 xyulose-5-P 10 G-3-P 2 G-3-P 2 erythrose-4-P 2 DHAP 2 sedoheptulose-7-P 2 G-3-P 2 xylulose-5-P 2 ribose-5-P 6 ribose-5-P + 6 ATP 6 RuBP + 6 ADP
Regeneration of RuBP
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Why is cyclic photophosphorylation needed?
• Non-cycl ic photophosphorylation make . s ca one ATP per NADPH
• Calvin cycle uses 18 ATP pe 12r NADPH
• Cyclic photophosphorylation makes the re st of the ATP
Why is Rubisco called a “carboxylase-oxygenase?
- Alternative activity at high temperatures:
O2 O2
CH2OPO32-
CO2- phosphoglycolate
+
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The mode of action of many herbicides involves photosynthesis
DCMU binds to the Dsubunit of PSII,blocks electronflow to Q, and thus cuts the supply of NADPH
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The mode of action of many herbicides involves photosynthesis
Paraquat acceptselectrons from PSIand transfers them one at a time to O2 to form superoxide(O2
-), which thenforms H2O2, an oxidizing agent thatdisrupts membranes.
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Summary:
•Light reactions reduce NADP+ to NADPH.•Light reactions also phosphorylate ADP to ATP.•Rubisco incorporates CO2 into organic compound: PGA.•Dark reactions use NADPH and ATP to force reduction of PGA.
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