Carbon fixation reactions of photosynthesis : C from CO 2 is fixed into biological molecules

1. Carbon fixation reactions of photosynthesis: C from CO2 is fixed into biological molecules major pathway for this is the Calvin cycle (C3 photosynthetic pathway) first reaction in this pathway: ribulose bisphosphate + CO2 + H2O two 3-phosphoglyceric acid (RuBP) (3-PGA) catalyzed by RuBP carboxylase

2. - two molecules of 3-phosphoglyceric acid are produced - Calvin cycle continues

3. carboxylation stage of the Calvin cycle ---------------------------------------------------------------------------------------------------- reduction stage of the Calvin cycle occurs in two reactions: 1) 3-PGA + ATP bisphosphoglycerate (bisPGA) 2) bisPGA + NADPH glyceraldehyde-3-phosphate + NADP+ (G-3-P) ** G-3-P is a “reduced carbon” molecule --------------------------------------------------------------------------------------------------------------- regeneration stage of the Calvin cycle - many reactions resulting in regeneration of RuBP

6. can account for having G-3-P used in carbohydrate synthesis and in RuBP regeneration by looking at a balanced equation of these stages: Carboxylation: 3 RuBP + 3 CO2 + 3 H2O 6 3-PGA (15 C) (3 C) (18 C) Reduction: 6 3-PGA + 6 ATP 6 bisPGA + 6 ADP (18 C) (18 C) 6 bisPGA + 6 NADPH 6 G-3-P + 6 NADP+ (18 C) (18 C) Regeneration: 5 G-3-P3 RuBP (15 C)(15 C)

7. the net product of the Calvin cycle is one molecule of G-3-P most often used to synthesize either starch or sucrose ** Calvin cycle occurs in the stroma portion of the chloroplast uses ATP and NADPH produced at lamellae location of starch synthesis location of sucrose synthesis

8. RuBP carboxylase also catalyzes a competing reaction RuBP + O2 3 - PGA + phosphoglycolate (5 C) (3 C) (2 C) - enzyme acts as an oxygenase (RuBP oxygenase) RuBP carboxylase/oxygenase (Rubisco)

9. the two reactions - compete for the same substrate - catalyzed by the same enzyme depends mostly on the relative concentrations of CO2 and O2: 1) Rubisco has a much greater affinity for CO2 than for O2 2) atmospheric O2 concentration ~ 21%; atmospheric CO2 concentration ~ 0.035% net effect: RuBP combines with CO2 3 - 4 times as often as with O2

10. combination of RuBP with O2 triggers the loss of reduced C - photorespiration(C2 oxidative photosynthetic carbon cycle) products of the initial oxygenation reaction are: 1) 3 - PGA 2) phosphoglycolate

11. 3 - PGA used to regenerate RuBP phosphoglycolate enters the glycolate pathway for every two molecules of phosphoglycolate: one CO2 is lost one 3 - PGA is produced

12. reactions of the C2 oxidative photosynthetic pathway - involve amino acids as participants - another result is the loss of an amino group from one of these amino acids - loss of this NH4+ is energy wasteful

13. another result is release of H2O2 as a by-product - requires that this step occur in a peroxisome -process also uses an ATP - advantage of pathway is to retrieve the C from phosphoglycolate Photorespiration is a light-dependent process - light activation of Rubisco - light effects on O2 concentration in the leaf