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Dark Reaction Topics

Dark Reaction Topics. CO 2 Fixation. Calvin Cycle. Starch and Sucrose Biosynthesis. C3 vs C4 Plants. Exergonic. C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O  G o’ = -2,860 kJ/mol. Endergonic. 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2  G o’ = + 2,860 kJ/mol.

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Dark Reaction Topics

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  1. Dark Reaction Topics CO2 Fixation Calvin Cycle Starch and Sucrose Biosynthesis C3 vs C4 Plants

  2. Exergonic C6H12O6 + 6O2 6CO2 + 6H2O Go’= -2,860 kJ/mol Endergonic 6CO2 + 6H2O C6H12O6 + 6O2Go’ = + 2,860 kJ/mol Dark Reaction Glucose Oxidation Dark Reaction

  3. CO2 Fixation (One Enzyme) CO2  electrons HO- CH2OP CH2OP CH2OP CH2OP Enediol C=O C-OH C-CO2 C- C-OH C=O C-OH C-OH C-OH C-OH C-OH C-OH CH2OP CH2OP CH2OP CH2OP CH2OP CH2OP .. H HO- HO- 2, 3PGA C-CO2 C-CO2 + HO- H HO- C-OH COO- HO- H H C-OH C-OH CH2OP CH2OP CO2 Ribulose 1,5bisPO4 H2O H+ or RUBISCO Ribulose 1,5 bisPO4 carboxylase

  4. The Calvin Cycle in Photosynthesis Rule: The Calvin cycle is the mechanism for turning CO2 into glucose Rule: The cycle fixes 6 CO2 to 6 ribulose 1,5-bisPO4 acceptor molecules, makes glucose with 6 of the carbons, and returning the rest to continue the cycle. Rule: The driving force is to synthesize the 5 carbon product ribulose 1,5-bisPO4

  5. 6CO2 Glucose 6CO2 + 6RuBP G6P + 6RuBP 6CO2 + 6RuBP 12, 3PGA Calvin Cycle Strategy (Ideal) (Real) 6C 30C 6C 30C (Mechanism) 6C 12, 3PGA G6P + 2F6P + 4G3P + 2DHAP 2E4P + 2SHBP 4Xu5P + 2Ru5P Intermediates 20C 10C Final

  6. ATP ADP NADPH + H+ NADP+ DHAP Carbon Balance in Calvin Cycle 12 3-PGA 36c 36c 12 1,3 bisPO4 glycerate 24c 6c 12 glyceraldehyde-3-PO4 Calvin Calvin 3 Fructose 1,6bisPO4 18c 12c 2 Fructose-6-PO4 Calvin 12c Glucose-6-PO4 6c

  7. C C=O TK C C C C CHO CHO C C=O C=O C C C C Ru5P C HO HO CH2OP CH2OP CH2OP CH2OP C CH2OP Pi CHO COH CHO HO TK CH2OP CH2OP C C C C=O C=O C C C TA C CH2OH C CH2OP CH2OP CH2OP Ru5P TK and TA Reactions F6P G3P Xu5P + Xu5P R5P DHAP S1,7BP E4P G3P

  8. Rule: In the Calvin cycle 5C sugars are never donor or acceptor substrates for transketolases or transaldolases CH2OH C=O CHO CH2OH CH2OH CH2OP HO-C C-OH C=O C=O C=O C-OH C-OH C-OH C-OH C-OH CH2OP C-OH C-OH C-OH C-OH CH2OP CH2OP CH2OP CH2OP Finishing Touches with 5 Carbon ATP Epimerase Kinase Xu5P Ru5P RuBP Isomerase ATP 30 carbons are returned to CO2 fixation R5P Ru5P

  9. G3P DHAP 3C G3P 3C 3C 3C G3P DHAP 3C 6C 4C 7C E4P SBP F6P 5C 5C 5C Xu5P R5P Xu5P

  10. UTP ATP PPi PPi fructose-6-PO4 (glucose)n H2O Starch (amylose) Pi Sucrose Synthesis of Sucrose and Starch Cytosol F6P  G6P  G1P ADP-glucose UDP-glucose ADP + (glucose)n+1 UDP + Sucrose-6-PO4 Stroma

  11. Thank goodness for photosynthesis

  12. Control of the Calvin Cycle ΔGo’ ΔG Phosphoribokinase -21.8 -15.9 Ribulose bisPO4 carboxylase -35.1 -41.0 Phosphoglycerate kinase + +18.0 -6.7 glyceraldehyde-3-PO4 dehydrogenase Triose phosphate isomerase -7.5 -0.8 Aldolase -21.8 -1.7 Fructose bisphosphatase -14.2 -27.2 Transketolase +6.3 -3.8 Aldolase -23.4 -0.8 Sedoheptulose bisphosphatase -14.2 -29.7 Transketolase +0.4 -5.9 Phosphopentose epimerase +0.8 -0.4 Ribose phosphate isomerase +2.1 -0.4 Each controlling enzyme is itself regulated by light either through pH or Mg+2

  13. Control of Flux in Plants Glyceraldehyde-3-PO4 Pi NADP+ Glyceraldehyde-3-PO4 dehydrogenase NADPH 1,3-Bisphosphoglycerate Does the Calvin cycle occur in the dark? ADP Phosphoglycerate kinase ATP 3-PGA NO Calvin Cycle

  14. CH2OPO3 HO-C-CO2 H-C-OH H-C-OH CH2OPO3 Thioredoxin 1,7 BisPtase FdOX FdRRED PS1*RED SH SH SH SH Thioredoxin PS1OX 1,7 BisPtase FdRED FdROX SH SH S S S S Binds tightly to RUBISCO when there is no light Therefore CO2 fixation cannot occur in the dark 2-Carboxyarabitol-1-PO4 Active* Inactive Sedoheptulose 1,7-bisphosphatase is activated by light Text p555

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