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Dark Reaction Topics CO2 Fixation Calvin Cycle

Published byJosephine Banks Modified over 8 years ago

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Presentation on theme: "Dark Reaction Topics CO2 Fixation Calvin Cycle"— Presentation transcript:

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

2 Dark Reaction Glucose Oxidation Exergonic Endergonic Dark Reaction
C6H12O O CO H2O Go’= -2,860 kJ/mol Dark Reaction 6CO H2O C6H12O O2 Go’ = + 2,860 kJ/mol

3 CO2 Fixation (One Enzyme)
CH2OP C=O C-OH Enediol  electrons CH2OP C-OH CO2 CH2OP C- C=O C-OH HO- CO2 Ribulose 1,5bisPO4 H2O CH2OP C-CO2 HO- H .. CH2OP C-CO2 HO- CH2OP C-CO2 C-OH HO- H H+ 2, 3PGA C-OH CH2OP HO- + H COO- C-OH CH2OP 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 Calvin Cycle Strategy 6CO2 Glucose 6CO2 + 6RuBP G6P + 6RuBP
(Ideal) 6CO2 Glucose (Real) 6CO2 + 6RuBP G6P + 6RuBP 6C 30C 6C 30C (Mechanism) 6CO2 + 6RuBP 12, 3PGA 6C 12, 3PGA G6P + 2F6P + 4G3P + 2DHAP 2E4P + 2SHBP 4Xu5P + 2Ru5P Intermediates 20C 10C Final

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

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

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

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

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

11 Thank goodness for photosynthesis

12 Control of the Calvin Cycle
ΔGo’ ΔG Phosphoribokinase Ribulose bisPO4 carboxylase Phosphoglycerate kinase glyceraldehyde-3-PO4 dehydrogenase Triose phosphate isomerase Aldolase Fructose bisphosphatase Transketolase Aldolase Sedoheptulose bisphosphatase Transketolase Phosphopentose epimerase Ribose phosphate isomerase Each controlling enzyme is itself regulated by light either through pH or Mg+2

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

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

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