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

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

2 2 Lecture Outline 1. 1.The Calvin Cycle fixes carbon makes reduced carbon compounds 2. 2.Reactions of the Calvin Cycle – anabolic pathway input of NADPH + H +, input of ATP 3. 3.Regulation of the Calvin Cycle 4. 4.The problem with oxygen – Photorespiration 5. 5.Tricks some plants use to limit photorespiration - C4 anatomy, C4 metabolism – division of labor - CAM plants, the difference is night and day

3 3 DARK REACTIONS energy utilization The Calvin Cycle

4 4 The purpose of the Carbon-fixation (Calvin Cycle) Reactions CO 2 + NADPH + H + + ATP C 6 H 12 O 6 + NADP + + ADP + P i carbohydrate Note: synthesis of carbohydrate from CO 2 is favorable only because coupled to very favorable reactions NADPH to NADP + and ATP to ADP + P i energy released is greater than it costs to make carbohydrate

5 5 The Calvin cycle has three phases –Carbon fixation –Reduction (energy input, reducing equiv input) –Regeneration of the CO 2 acceptor (energy input – “priming step”)

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

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

8 8 Acid -COOH Aldehyde -C=O -C=O H PrimingStep Input of energyRegenerate What started withCarbonFixationReduction Rubisco

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

10 10 Integration of Light-Dependent and Light-Independent Reactions They generally occur AT THE SAME TIME O2O2 CO 2 H2OH2O Light Light reaction Calvin cycle NADP + ADP ATP NADPH + P 1 RuBP 3-Phosphoglycerate Amino acids Fatty acids Starch (storage) Sucrose (export) G3P Photosystem II Electron transport chain Photosystem I Chloroplast Figure 10.21

11 11 PhotoRespiration - “ the OXYGEN PROBLEM” Oxygen Oxygen is a competing substrate for the 1st enzyme in the C 3 cycle (Rubisco) O2O2O2O2 5 Carbon Sugar with 2 phosphates on it Acid 3 Carbon Acid with one phosphate on it + 2Acid 2 Carbon Acid with one phosphate on it Energy Wasted With NO synthesis of glucose CO 2 5 Carbon Sugar with 2 phosphates on it Acid 3 Carbon Acid with one phosphate on it + Acid 3 Carbon Acid with one phosphate on it Net increase in material make glucose with “extra”

12 12 Some Plants Deal with this problem by a DIVISION OF LABOR BETWEEN CELLS C 4 Plants Mesophyll cells perform “usual” noncyclic Light-Dependent Reactions make oxygen, ATP and NADPH Do NOT perform the C 3 (Calvin cycle) reactions Bundle Sheath cells perform “UNusual” cyclic Light-Dependent Reactions a lot of ATP but very little NADPH and very little O 2 and very little O 2 PERFORM the usual C 3 (Calvin Cycle) reactions

13 13 C 4 leaf anatomy and the C 4 pathway CO 2 Mesophyll cell cell Bundle- sheathcell Vein (vascular tissue) Photosynthetic cells of C 4 plant leaf Stomata Mesophyll cell C 4 leaf anatomy PEP carboxylase Oxaloacetate (4 C) PEP (3 C) Malate (4 C) ADP ATP Bundle- Sheath cell CO 2 Pyruate (3 C) CALVIN CYCLE Sugar Vascular tissue Figure CO 2 Mesophyll Produce NADPH and ATP PEP carboxylase insensitive to O 2 Malate brings across reducing equivalents NADPHused NADPHregenerated Cyclic e- flow Little O 2 Malate (4 C ) Oxaloacetate (4 C)

14 14 CO 2 CH 3 C=O \ O-P i “PEP” “PEP” OH / C=O CH 2 C=O C=O + P i \ OHOxaloacetate 4C acid 4C acid OH / C=O CH 2 H--H H-C-O-H C=O \ OH malate malate 4C acid 4C acid CH 3 C=O \ O-Hpyruvate ATP ADP NADPH+ H + NADP + PEP carboxylase carries” “carries”Reducingequivalents Mesophyll Cell C4 Metabolism Needs ATP and NAPDPH + H + Non-cyclic electron flow

15 15 OH / C=O CH 2 C=O \ OHOxaloacetate 4C acid 4C acid OH / C=O CH 2 H--H H-C-O-H C=O \ OH malate malate 4C acid 4C acidNADPH+ H + NADP + carries” “carries”Reducingequivalents Bundle Sheath - Cell C4 Metabolism ATP Needs ATP Only Cyclic electron flow Very low O 2 CO 2 CH 3 C=O \ O-Hpyruvate ATP CalvinCycle glucose ATP NADPH

16 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 C 3 reactions Costs more energy to do business this way… but has the advantage when CO 2 is limiting (when stomates are closed - like on hot days) Who cares as long as the sun is shining ? ATP is not limiting

17 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 (CO 2 assimilation) accumulate malate to high concentration in central vacuole use sugar oxidation/catabolism to power (NADH and ATP) carbon fixation DAY Perform “light” reactions during the day mostly cyclic e - flow to produce ATP (low O 2 ) decarboxylate malate to yield CO 2 and NADPH + H + perform C3 reactions (Calvin Cycle) to produce sugars and starch

18 18 Spatial separation of steps. In C 4 plants, carbon fixation and the Calvin cycle occur in different types of cells. (a) Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cells at different times. (b) Pineapple Sugarcane Bundle- sheath cell Mesophyll Cell Organic acid CALVIN CYCLE Sugar CO 2 Organic acid CALVIN CYCLE Sugar C4C4 CAM CO 2 incorporated into four-carbon organic acids (carbon fixation) Night Day 1 2 Organic acids release CO 2 to Calvin cycle Figure SunlightPowersBothphases SugarOxidationPowersOnePhase GenerallySlowgrowing

19 19Summary 1. 1.Photosynthetic “light” reactions produce ATP and reducing potential NADPH + H + ATP 2. Dark reactions use ATP and reducing potential to synthesize carbohydrates reductionacid - powers reduction of 3-carbon acid aldehyde to 3-carbon aldehyde - - powers regeneration of starting material 5-carbon di-phosphate (priming step for CO 2 fixation) tightly 3. Rubisco enzyme regulated tightly by allosteric modulators pH, and reducing status of stroma O 2 4. O 2 interferes with carbon fixation by Rubisco enzyme 5. Metabolic “tricks” to avoid photorespiration C4 metabolismCAM metabolism - C4 metabolism - CAM metabolism


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