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Lecture 17 Oct 10, 2005 Photosynthesis II. Calvin Cycle

Lecture 17 Oct 10, 2005 Photosynthesis II. Calvin Cycle. Lecture Outline The Calvin Cycle fixes carbon makes reduced carbon compounds Reactions of the Calvin Cycle – anabolic pathway input of NADPH + H + , input of ATP Regulation of the Calvin Cycle

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Lecture 17 Oct 10, 2005 Photosynthesis II. Calvin Cycle

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  1. Lecture 17 Oct 10, 2005 Photosynthesis II. Calvin Cycle

  2. Lecture Outline • The Calvin Cycle fixes carbon • makes reduced carbon compounds • Reactions of the Calvin Cycle – anabolic pathway • input of NADPH + H+, input of ATP • Regulation of the Calvin Cycle • The problem with oxygen – Photorespiration • Tricks some plants use to limit photorespiration • - C4 anatomy, C4 metabolism – division of labor • - CAM plants, the difference is night and day

  3. DARK REACTIONS energy utilization The Calvin Cycle

  4. The purpose of the Carbon-fixation (Calvin Cycle) Reactions CO2 + NADPH + H+ + ATP C6H12O6 + NADP+ + ADP + Pi carbohydrate Note: synthesis of carbohydrate from CO2 is favorable only because coupled to very favorable reactions NADPH to NADP+ and ATP to ADP + Pi energy releasedis greater than it costs to make carbohydrate

  5. The Calvin cycle has three phases • Carbon fixation • Reduction (energy input, reducing equiv input) • Regeneration of the CO2 acceptor (energy input – “priming step”)

  6. Overview of Carbon-Fixation Reactions O = R-C-OH 3 Carbon Acid with one phosphate on it 2 ATP Reduction ADP + Pi ADP + Pi NADPH ATP NADP+ O 5 Carbon Sugar with one phosphate on it = R-C-H Higher Energy Compound 6 Carbon Sugar - Glucose with NO phosphate on it 12 24 CO2 + 5 Carbon Sugar with 2 phosphates on it 12 24 20/24 3 Carbon Aldehyde with one phosphate on it 4/24 2

  7. Carbon Fixation Carried out by the enzyme “rubisco” (ribulose 1,5 bisphosphate carboxylase oxygenase) Do need to know this enzyme Key regulatory enzyme

  8. Carbon Fixation Acid -COOH Input of energy Reduction Rubisco Priming Step Aldehyde -C=O H Regenerate What started with

  9. Regulation of Rubisco 1st Enzyme in Calvin Cycle Substrate/Productavailability Allosterically regulated by NADPH and ATP Very Narrow pH optimum pH 8 / pH7 pH 8 or above, inactive at 7 Enzyme must be in reduced form

  10. Light reaction Calvin cycle H2O CO2 Light NADP+ ADP + P1 RuBP 3-Phosphoglycerate Photosystem II Electron transport chain Photosystem I ATP G3P Starch (storage) NADPH Amino acids Fatty acids Chloroplast O2 Sucrose (export) Integration of Light-Dependent and Light-Independent Reactions They generally occur AT THE SAME TIME Figure 10.21

  11. 3 Carbon Acid with one phosphate on it CO2 + PhotoRespiration - “ the OXYGEN PROBLEM” Oxygen is a competing substrate for the 1st enzyme in the C3 cycle (Rubisco) 3 Carbon Acid with one phosphate on it 5 Carbon Sugar with 2 phosphates on it 2 Carbon Acid with one phosphate on it O2 + 3 Carbon Acid with one phosphate on it 5 Carbon Sugar with 2 phosphates on it Net increase in material make glucose with “extra” Energy Wasted With NO synthesis of glucose

  12. Some Plants Deal with this problem by a DIVISION OF LABOR BETWEEN CELLS C4 Plants Mesophyll cells perform “usual” noncyclic Light-Dependent Reactions make oxygen, ATP and NADPH DoNOT perform the C3 (Calvin cycle) reactions Bundle Sheath cells perform “UNusual” cyclic Light-Dependent Reactions a lot of ATP but very little NADPH and very little O2 PERFORM the usual C3 (Calvin Cycle) reactions

  13. Mesophyll cell Mesophyll cell Photosynthetic cells of C4 plant leaf CO2 CO2 PEP carboxylase Bundle- sheath cell C4 leaf anatomy and the C4 pathway PEP (3 C) Oxaloacetate (4 C) ADP NADPH used Vein (vascular tissue) Malate (4 C) ATP Malate (4 C) C4 leaf anatomy Pyruate (3 C) Oxaloacetate (4 C) NADPH regenerated CO2 Mesophyll Produce NADPH and ATP PEP carboxylase insensitive to O2 Malate brings across reducing equivalents Stomata Bundle- Sheath cell Cyclic e- flow Little O2 CALVIN CYCLE Sugar Vascular tissue Figure 10.19

  14. Mesophyll Cell C4 Metabolism PEP carboxylase NADPH + H+ NADP+ ADP ATP CO2 OH / C=O CH2 C=O C=O +Pi \ OH Oxaloacetate 4C acid OH / C=O CH2 H-C-O-H C=O \ OH malate 4C acid CH3 C=O C=O \ O-Pi “PEP” CH3 C=O C=O \ O-H pyruvate “carries” Reducing equivalents Needs ATP and NAPDPH + H+ Non-cyclic electron flow

  15. Bundle Sheath - Cell C4 Metabolism CH3 C=O C=O \ O-H pyruvate NADPH + H+ NADP+ CO2 NADPH Calvin Cycle ATP ATP glucose OH / C=O CH2 H-C-O-H C=O \ OH malate 4C acid OH / C=O CH2 C=O C=O \ OH Oxaloacetate 4C acid “carries” Reducing equivalents Needs ATP Only Cyclic electron flow Very low O2

  16. Mesophyll cells provide a means for bundle sheath cells to acquire NADPH + H+ reducing power Mesophyll cells provide carbon dioxide to bundle sheath cells at higher concentration than in air Bundle Sheath cells not making oxygen, so very little competitor with C3 reactions Costs more energy to do business this way… but has the advantage when CO2 is limiting (when stomates are closed - like on hot days) Who cares as long as the sun is shining ? ATP is not limiting

  17. CAM Plants Cacti, pineapple • Open their stomata only at night, too hot during day – survive very adverse (dry) conditions NIGHT Perform PEP carboxylase reaction at night (CO2 assimilation) accumulate malateto high concentration in central vacuole use sugar oxidation/catabolism to power(NADH andATP) carbon fixation DAY Perform “light” reactions during the day mostly cyclic e- flow to produce ATP(low O2) decarboxylate malate to yield CO2 and NADPH + H+ perform C3 reactions (Calvin Cycle) to produce sugars and starch

  18. 1 2 Pineapple Sugarcane C4 CAM CO2 CO2 Mesophyll Cell Night Organic acid Organic acid CO2 incorporated into four-carbon organic acids (carbon fixation) Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cellsat different times. Bundle- sheath cell (b) Day Spatial separation of steps. In C4 plants, carbon fixation and the Calvin cycle occur in different types of cells. (a) CALVINCYCLE CALVINCYCLE Organic acids release CO2 to Calvin cycle Sugar Sugar Figure 10.20 Generally Slow growing Sugar Oxidation Powers One Phase Sunlight Powers Both phases

  19. Summary • Photosynthetic “light” reactions produce • ATP andreducing potential NADPH + H+ • 2. Dark reactions use ATP and reducing potential • to synthesize carbohydrates • - powers reduction of 3-carbon acid • to 3-carbon aldehyde • - powers regeneration of starting material • 5-carbon di-phosphate (priming step for CO2 fixation) • 3. Rubisco enzyme regulatedtightlyby allosteric modulators • pH, and reducing status of stroma • 4.O2 interferes with carbon fixation by Rubisco enzyme • 5. Metabolic “tricks” to avoid photorespiration • - C4 metabolism - CAM metabolism

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