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Calvin Cycle Photosynthesis: Carbon Reactions 1. Calvin cycle 1. 2. C2 Oxidative Photosynthetic Carbon Cycle or Photorespiration 3. 3. CO2 Concentrating Mechanisms CO2 Pumping C4 Photosynthesis Crassulacean Acid Metabolism (CAM) Carboxylation = CO2 Fixation 2. Reduction of 3-phosphoglycerate Rubisco 1. ATP phosphorylates 3-phosphoglycerate Ribulose-1,5-bisphosphate carboxylase/oxygenase 1. 40% of chloroplast protein 2. 500:1 ratio of enzyme to CO2 2. NADPH reduces 1,3-phosphoglycerate to Glyceraldehyde 3-phosphate Regeneration of Ribulose 1,5-bisphosphate Unraveling the Calvin Cycle Rubulose 1,5-bisphosphate 3 5 6 5 5 6 3 4 5 7 5 3 7 5 1 Calvin Cycle Summary 1. Autocatalytic – makes more substrate (RuBP) than consumed 2. Fate of triosphophate: 5/6 Regeneration of RuBP 1/6 exported for sucrose synthesis 3. To fix 6 molecules of CO2 requires 18 ATP 12 NADPH 4. Energy efficiency of the Calvin Cycle based on free energies 1 mole hexose = 2804 kJ (2016 cal) 12 NADPH = (12 X 217) = 2804 kJ (624 cal) from NADPH 18 ATP = (18 X 29) = 522 kJ (126 cal) from ATP = 3126 kJ (750 cal) from NADPH & ATP Efficiency = energy stored / energy used = 2804 / 3126 ≈ 90% 5. Sources of Energy: Energy: 6. Thermodynamic efficiency: END Calvi n Cycle 83% from NADPH 17% from ATP = 2804 kJ hexose / 8400 kJ light energy ≈ 33% C2 Oxidative Photosynthetic Carbon Cycle & Photorespiration Photorespiratory Cycle 1. Three organelles involved 1. Rubisco catalyzes oxygenation and carboxylation of RuBP A. When [CO2] = [O2]… CO2 fixed 80X faster than O2 B. In air [CO2] << [O2]… CO2 fixed 3X faster than O2 2. Photorespiration is oxygen consumption and oxygenation of RuBP 3. C2 oxidative photosynthetic cycle results in partial recovery of fixed Carbon Photorespiratory Cycle Chloroplast Photorespiratory Cycle 2 Glycolates from Chloroplast Peroxisome 2 Glycolates leave Chloroplast 2 Glycines Leave Peroxisome 2 Photorespiratory Cycle Photorespiratory Cycle αketoglutarate Out to Chloroplast Ammonium Nitrogen returns to Chloroplast 2 Glycines from Peroxisome Glycerate Out to Chloroplast Serine Leave Mitochondrion Mitochondrion One CO2 Lost Two 2-Carbon Amino Acid Glycines One 3-Carbon Amino Acid Serine Peroxisome Serine In Photorespiratory Cycle Photorespiratory Cycle Chloroplast NH4+ from mitochondrion Glutamate to Peroxisome αkGlutarate Glycerate From Peroxisome Consequences of Photorespiration CO2 Concentration Mechanisms 1. Loss of 25% carbon fixed in photosynthesis 2 glycine (2c amino acid) Æ 1 serine (3c amino acid) + 1 CO2 2. Photorespiration lowers efficiency of photosynthesis from 90% to 50%. 1. CO2 Pumps 2. C4 Photosynthetic Carbon Fixation 3. Crassulacean Acid Metabolism (CAM) 3. Warmer temperature reduce CO2 solubility faster than O2. Implications: A. Increasing temp Æ Increasing Photorespiration 4. Function of Photorespiration A. Dissipation of excess energy under high light B. Recapture 75% carbon potentially lost (from glycolate). 3 Carbon Dioxide and Bicarbonate Pumps C4 Photosynthetic Carbon Fixation In cyanobacteria and algae Suppresses photorespiration 1. Adaptation to minimize Photorespiration in high light, low water conditions 2. Requires two CO2 fixations spatially separated in the leaf CO2 gas channel 3. Four stages 1. Fixation of CO2 into a 4 Carbon acid in mesophyll by PEP 2. Transport of C4 acid to bundle sheath cells 3. Decarboxylation of C4 acid & CO2 generation & Calvin Cycle fixation of CO2 4. Transport of C3 acid back to mesophyll cells & PEP synthesis C4 Photosynthetic Carbon Fixation C4 Photosynthetic Carbon Fixation Leaf Anatomy Mesophyll Cells C4 Photosynthetic Carbon Fixation Consequences & Occurrence 1. Suppresses photorespiration a. O2 does not compete with PEP carboxylase b. Concentrated CO2 in Bundle Sheath cells 2. Minimizes water loss due to reduced stomatal aperture and steeper CO2 gradient 3. Energy Cost: 2 ATP per CO2 fixed. 4. C4 plants often found in hotter, drier climates Found in 1% of all known species… …Tropical grasses, sugarcane, maize (corn) Bundle Sheath CAM 1. Crassulacean Acid Metabolism 2. Observed in Crassulaceae (Jade plant, Kalanchoe) Cacti Euphorbia Pineapple, vanilla (orchid), agave 3. Function: Enhanced water use efficiency Stomata open at night & remain closed during daylight C3 Plant: loses 400-500g H2O per gram C fixed CAM Plant: loses 50-100 g H2O per gram C fixed 4. Temporal and spatial separation of two CO2 fixations 4 CAM during the Day CAM at Night 3C 4C 3C 4C 4C CAM Photosynthetic Carbon Fixation Consequences & Occurrence END Photosynthetic Carbon Reactions Photorespiration & C2 Pathway C4 & CAM 1. Adaptation to hot, dry conditions often for extended periods 2. May be a facultative adaptation to stressful conditions of heat or water e.g. Ice plant Unstressed Æ C3 Stressed Æ CAM 5